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library("petersenlab")
library("rms")
library("car")
library("caret")
library("lme4")
library("performance")
library("lavaan")
library("mice")
library("miceadds")
library("interactions")
library("brms")
library("robustbase")
library("ordinal")
library("MASS")
library("broom")
library("tidyverse")
library("knitr")We created the player_stats_weekly.RData and player_stats_seasonal.RData objects in Section 5.4.3.
Multiple regression is an extension of correlation. Correlation examines the association between one predictor variables and one outcome variable. Multiple regression examines the association between multiple predictor variables and one outcome variable. It allows obtaining a more accurate estimate of the unique contribution of a given predictor variable, by controlling for other variables (covariates).
Regression with one predictor variable takes the form of Equation 12.1:
\[ y = \beta_0 + \beta_1x_1 + \epsilon \tag{12.1}\]
where \(y\) is the outcome variable, \(\beta_0\) is the intercept, \(\beta_1\) is the slope, \(x_1\) is the predictor variable, and \(\epsilon\) is the error term.
A regression line is depicted in Figure 12.29.
Regression with multiple predictors—i.e., multiple regression—takes the form of Equation 12.2:
\[ y = \beta_0 + \beta_1x_1 + \beta_2x_2 + ... + \beta_px_p + \epsilon \tag{12.2}\]
where \(p\) is the number of predictor variables. Multiple regression is basically a weighted sum of the predictor variables, and adding an intercept. Under the hood, multiple regression seeks to identify the best weight for each predictor.
The unstandardized regression coefficient (\(B\)) is interpreted such that, for every unit change in the predictor variable, there is a __ unit change in the outcome variable. For instance, when examining the association between age and fantasy points, if the unstandardized regression coefficient is 2.3, players score on average 2.3 more points for each additional year of age. (In reality, we might expect a nonlinear, inverted-U-shaped association between age and fantasy points such that players tend to reach their peak in the middle of their careers.) Unstandardized regression coefficients are tied to the metric of the raw data. Thus, a large unstandardized regression coefficient for two variables may mean completely different things. Holding the strength of the association constant, you tend to see larger unstandardized regression coefficients for variables with smaller units and smaller unstandardized regression coefficients for variables with larger units.
Standardized regression coefficients can be obtained by standardizing the variables to z-scores so they all have a mean of zero and standard deviation of one. The standardized regression coefficient (\(\beta\)) is interpreted such that, for every standard deviation change in the predictor variable, there is a __ standard deviation change in the outcome variable. For instance, when examining the association between age and fantasy points, if the standardized regression coefficient is 0.1, players score on average 0.1 standard deviation more points for each additional standard deviation of their year of age. Standardized regression coefficients—though not the case in all instances—tend to fall between [−1, 1]. Thus, standardized regression coefficients tend to be more comparable across variables and models compared to unstandardized regression coefficients. In this way, standardized regression coefficients provide a meaningful index of effect size.
The standard error of a regression coefficient represents the uncertainty of the parameter. If we have less uncertainty (i.e., more confidence) about the parameter, the standard error will be small. If we have more uncertainty (i.e., less confidence) about the parameter, the standard error will be large. If we used the same sampling procedure repeatedly and calculated the regression coefficient each time, the true parameter in the population would fall 68% of the time within the interval of: \([\text{model parameter estimate for the regression coefficient} \pm 1 \text{ standard error}]\) .
A confidence interval represents a range of plausible values such that, with repeated sampling, the true value falls within a given interval with some confidence. Our parameter estimate for the regression coefficient, plus or minus 1 standard error, reflects the 68% confidence interval for the coefficient. The 95% confidence interval is computed as the parameter estimated plus or minus 1.96 standard errors. For instance, if the parameter estimate for the regression coefficient is 0.50, and the standard error is 0.10, the 95% confidence interval is [0.30, 0.70]: \(0.5 - (1.96 \times 0.10) = 0.3\); \(0.5 + (1.96 \times 0.10) = 0.7\). That is, if we used the same sampling procedure repeatedly, the true value of the regression coefficient would be expected to be 95% of the time somewhere between 0.30 to 0.70.
When the outcome variable is continuous, linear regression is common. However, there are other types of regression depending on type and distribution of the outcome variable. For instance, if the outcome variable is binary, logistic regression would be used. If the outcome variable is an ordered categorical variable, ordinal regression would be used. If the outcome variable is a count, Poisson or negative binomial regression would be preferable. If the outcome variable is a proportion, beta regression would be used.
Here are examples of each:
Call:
lm(formula = fantasyPoints ~ age + height + weight + target_share,
data = player_stats_seasonal %>% filter(position %in% c("WR")),
na.action = "na.exclude")
Residuals:
Min 1Q Median 3Q Max
-739.80 -18.68 -10.50 10.74 292.78
Coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) -34.62684 23.85029 -1.452 0.146617
age 0.75193 0.22418 3.354 0.000803 ***
height 0.05816 0.42094 0.138 0.890107
weight 0.14716 0.06686 2.201 0.027794 *
target_share 743.14867 7.38684 100.604 < 2e-16 ***
---
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
Residual standard error: 45.71 on 4432 degrees of freedom
(952 observations deleted due to missingness)
Multiple R-squared: 0.7054, Adjusted R-squared: 0.7052
F-statistic: 2654 on 4 and 4432 DF, p-value: < 2.2e-16# Standardization method: refit
Parameter | Std. Coef. | 95% CI
-----------------------------------------
(Intercept) | -8.91e-17 | [-0.02, 0.02]
age | 0.03 | [ 0.01, 0.04]
height | 1.63e-03 | [-0.02, 0.02]
weight | 0.03 | [ 0.00, 0.05]
target share | 0.83 | [ 0.82, 0.85]newdata <- player_stats_weekly %>%
mutate( # create a binary variable to be used as the outcome variable
receiving_td = case_when(
is.na(receiving_tds) ~ NA_real_, # keep NA
receiving_tds >= 1 ~ 1,
receiving_tds == 0 ~ 0
)
)
logisticRegression <- glm(
receiving_td ~ age + height + weight + target_share,
data = newdata %>% filter(position %in% c("WR")),
family = binomial(),
na.action = "na.exclude"
)
summary(logisticRegression)
Call:
glm(formula = receiving_td ~ age + height + weight + target_share,
family = binomial(), data = newdata %>% filter(position %in%
c("WR")), na.action = "na.exclude")
Coefficients:
Estimate Std. Error z value Pr(>|z|)
(Intercept) -6.4762075 0.4747551 -13.641 < 2e-16 ***
age -0.0043680 0.0041374 -1.056 0.291
height 0.0495091 0.0083609 5.922 3.19e-09 ***
weight 0.0008781 0.0012750 0.689 0.491
target_share 8.0628633 0.1242041 64.916 < 2e-16 ***
---
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
(Dispersion parameter for binomial family taken to be 1)
Null deviance: 42304 on 44307 degrees of freedom
Residual deviance: 37107 on 44303 degrees of freedom
(12044 observations deleted due to missingness)
AIC: 37117
Number of Fisher Scoring iterations: 5# Standardization method: refit
Parameter | Std. Coef. | 95% CI
------------------------------------------
(Intercept) | -1.72 | [-1.75, -1.69]
age | -0.01 | [-0.04, 0.01]
height | 0.12 | [ 0.08, 0.16]
weight | 0.01 | [-0.02, 0.05]
target share | 0.88 | [ 0.85, 0.90]
- Response is unstandardized.
0 1 2 3 4
420249 15050 1818 182 10 formula: receiving_tdsFactor ~ age + height + target_share
data: newdata %>% filter(position %in% c("WR"))
link threshold nobs logLik AIC niter max.grad cond.H
logit flexible 44308 -22202.80 44419.59 7(0) 5.34e-07 3.0e+07
Coefficients:
Estimate Std. Error z value Pr(>|z|)
age -0.004442 0.004092 -1.085 0.278
height 0.055903 0.005731 9.754 <2e-16 ***
target_share 8.171193 0.121637 67.177 <2e-16 ***
---
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
Threshold coefficients:
Estimate Std. Error z value
0|1 6.7753 0.4320 15.69
1|2 9.0558 0.4334 20.89
2|3 11.4826 0.4429 25.93
3|4 14.8144 0.6612 22.41
(12044 observations deleted due to missingness)# Standardization method: refit
Component | Parameter | Std. Coef. | 95% CI
------------------------------------------------------
intercept | 0|1 | 1.72 | [ 1.69, 1.75]
intercept | 1|2 | 4.00 | [ 3.94, 4.06]
intercept | 2|3 | 6.43 | [ 6.24, 6.61]
intercept | 3|4 | 9.76 | [ 8.78, 10.74]
location | age | -0.01 | [-0.04, 0.01]
location | height | 0.13 | [ 0.11, 0.16]
location | target share | 0.89 | [ 0.86, 0.91]
- Response is unstandardized.
Call:
glm(formula = receiving_tds ~ age + height + weight + target_share,
family = poisson(), data = newdata %>% filter(position %in%
c("WR")), na.action = na.exclude)
Coefficients:
Estimate Std. Error z value Pr(>|z|)
(Intercept) -5.9753891 0.3733793 -16.004 < 2e-16 ***
age 0.0001296 0.0032293 0.040 0.968
height 0.0459619 0.0065711 6.995 2.66e-12 ***
weight 0.0009397 0.0010008 0.939 0.348
target_share 5.1190772 0.0634014 80.741 < 2e-16 ***
---
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
(Dispersion parameter for poisson family taken to be 1)
Null deviance: 32902 on 44307 degrees of freedom
Residual deviance: 27962 on 44303 degrees of freedom
(12044 observations deleted due to missingness)
AIC: 45041
Number of Fisher Scoring iterations: 6# Standardization method: refit
Parameter | Std. Coef. | 95% CI
------------------------------------------
(Intercept) | -1.75 | [-1.77, -1.73]
age | 4.06e-04 | [-0.02, 0.02]
height | 0.11 | [ 0.08, 0.14]
weight | 0.01 | [-0.02, 0.04]
target share | 0.56 | [ 0.54, 0.57]
- Response is unstandardized.
Call:
MASS::glm.nb(formula = receiving_tds ~ age + height + weight +
target_share, data = newdata %>% filter(position %in% c("WR")),
na.action = na.exclude, control = glm.control(maxit = 10000),
init.theta = 4.750305306, link = log)
Coefficients:
Estimate Std. Error z value Pr(>|z|)
(Intercept) -5.7604913 0.3884147 -14.831 < 2e-16 ***
age -0.0022711 0.0033461 -0.679 0.497
height 0.0418343 0.0068311 6.124 9.12e-10 ***
weight 0.0008346 0.0010331 0.808 0.419
target_share 6.0065807 0.0793939 75.655 < 2e-16 ***
---
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
(Dispersion parameter for Negative Binomial(4.7503) family taken to be 1)
Null deviance: 30971 on 44307 degrees of freedom
Residual deviance: 25953 on 44303 degrees of freedom
(12044 observations deleted due to missingness)
AIC: 44851
Number of Fisher Scoring iterations: 1
Theta: 4.750
Std. Err.: 0.424
2 x log-likelihood: -44839.055 # Standardization method: refit
Parameter | Std. Coef. | 95% CI
------------------------------------------
(Intercept) | -1.80 | [-1.82, -1.77]
age | -7.12e-03 | [-0.03, 0.01]
height | 0.10 | [ 0.07, 0.13]
weight | 0.01 | [-0.02, 0.04]
target share | 0.65 | [ 0.63, 0.67]
- Response is unstandardized. Family: zero_one_inflated_beta
Links: mu = logit; phi = identity; zoi = identity; coi = identity
Formula: target_share ~ age + height + weight
Data: newdata %>% filter(position %in% c("WR")) (Number of observations: 44308)
Draws: 4 chains, each with iter = 2000; warmup = 1000; thin = 1;
total post-warmup draws = 4000
Regression Coefficients:
Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS Tail_ESS
Intercept -2.76 0.12 -3.00 -2.52 1.00 4499 3173
age 0.02 0.00 0.02 0.02 1.00 6652 2911
height -0.01 0.00 -0.01 -0.00 1.00 4661 2635
weight 0.01 0.00 0.00 0.01 1.00 4397 3278
Further Distributional Parameters:
Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS Tail_ESS
phi 12.24 0.09 12.07 12.42 1.00 2477 2367
zoi 0.13 0.00 0.13 0.14 1.00 2074 2328
coi 0.00 0.00 0.00 0.00 1.00 1641 2015
Draws were sampled using sampling(NUTS). For each parameter, Bulk_ESS
and Tail_ESS are effective sample size measures, and Rhat is the potential
scale reduction factor on split chains (at convergence, Rhat = 1).Linear regression models make the following assumptions:
Homoscedasticity of the residuals means that the variance of the residuals does not differ as a function of the outcome variable or as a function of the predictor variable (i.e., the residuals show constant variance as a function of outcome/predictors). If the residuals differ as a function of the outcome or predictor variable, this is called heterotoscedasticity.
Those are some of the key assumptions of multiple regression. However, there are additional assumptions of multiple regression, including ones discussed in the chapter on Causal Inference. For instance, the variables included should reflect a causal process such that the predictor variables influence the outcome variable, and not the other way around. That is, the outcome variable should not influence the predictor variables (i.e., there should be no reverse causation). In addition, it is important control for any confound(s). If a confound is not controlled for, this is called omitted-variable bias, and it leads the researcher to incorrectly attribute the effects of the omitted variable to the included variables.
To evaluate the shape of the association between the predictor variables and the outcome variable, we can examine scatterplots (Figure 12.2), residual plots (Figure 12.21), marginal model plots (Figure 12.12), added-variable plots (Figure 12.13), and component-plus-residual plots (Figure 12.14). Residual plots depict the residuals (errors) on the y-axis as a function of the fitted values or a specific predictor on the x-axis. Marginal model plots are basically glorified scatterplots that depict the outcome variable (both in terms of observed values and model-fitted values) on the y-axis and the predictor variables on the x-axis. Added-variable plots depict the unique association of each predictor variables with the outcome variable when controlling for all the other predictor variables in the model. Component-plus-residual plots depict partial residuals on the y-axis as a function of each predictor variable on the x-axis, where a partial residual for a given predictor is the effect of a given predictor (thus controlling for all the other predictor variables in the model) plus the residual from the full model.
Examples of linear and nonlinear associations are depicted with scatterplots in Figure 12.2.
set.seed(52242)
sampleSize <- 1000
quadraticX <- runif(
sampleSize,
min = -4,
max = 4)
linearY <- quadraticX + rnorm(sampleSize, mean = 0, sd = 0.5)
quadraticY <- quadraticX ^ 2 + rnorm(sampleSize)
quadraticData <- cbind(quadraticX, quadraticY) %>%
data.frame %>%
arrange(quadraticX)
quadraticModel <- lm(
quadraticY ~ quadraticX + I(quadraticX ^ 2),
data = quadraticData)
quadraticNewData <- data.frame(
quadraticX = seq(
from = min(quadraticData$quadraticX),
to = max(quadraticData$quadraticY),
length.out = sampleSize))
quadraticNewData$quadraticY <- predict(
quadraticModel,
newdata = quadraticNewData)
plot(
x = quadraticX,
y = linearY,
xlab = "",
ylab = "",
main = "Linear Association")
abline(
lm(linearY ~ quadraticX),
lwd = 2,
col = "blue")
plot(
x = quadraticData$quadraticX,
y = quadraticData$quadraticY,
xlab = "",
ylab = "",
main = "Nonlinear Association")
lines(
quadraticNewData$quadraticY ~ quadraticNewData$quadraticX,
lwd = 2,
col = "blue")
If the shape of the association is nonlinear (as indicated by any of these plots), various approaches may be necessary such as including nonlinear terms (e.g., polynomial terms such as quadratic, cubic, quartic, or higher-degree terms), transforming the predictors (e.g., log, square root, inverse, exponential, Box-Cox, Yeo-Johnson transform), use of splines/piecewise regression, and generalized additive models.
To evaluate homoscedasticity, we can evaluate a residual plot (Figure 12.21) and spread-level plot (Figure 12.22). A residual plot depicts the residuals on the y-axis as a function of the model’s fitted values on the x-axis. Homoscedasticity in a residual plot is identified as a constant spread of residuals versus fitted values—the residuals do not show a fan, cone, or bow-tie shape; a fan, cone, or bow-tie shape indicates heteroscedasticity. In a residual plot, a fan or cone shape indicates increasing or decreasing variance in the residuals as a function of the fitted values; a bow-tie shape indicates that the residuals are smallest in the middle of the fitted values and greatest on the extremes of the fitted values. A spread-level plot depicts the log of the absolute value of studentized residuals on the y-axis as a function of the log of the model’s fitted values on the x-axis. Homoscedasticity in a spread-level plot is identified as a flat slope; a slope that differs from zero indicates heteroscedasticity.
set.seed(52242)
# 1. Homoscedasticity
sampleSize <- 1000
x1 <- runif(sampleSize, 0, 10)
y1 <- 3 + 2 * x1 + rnorm(sampleSize, mean = 0, sd = 2)
fit1 <- lm(y1 ~ x1)
res1 <- resid(fit1)
fitted1 <- fitted(fit1)
# 2. Fan-shaped heteroscedasticity
x2 <- runif(sampleSize, 0, 10)
y2 <- 3 + 2 * x2 + rnorm(sampleSize, mean = 0, sd = 0.5 * x2) # increasing variance
fit2 <- lm(y2 ~ x2)
res2 <- resid(fit2)
fitted2 <- fitted(fit2)
# 3. Bow-tie-shaped heteroscedasticity
x3 <- runif(sampleSize, 0, 10)
sd3 <- abs(x3 - 5) + 0.5 # variance smallest in middle, higher on edges
y3 <- 3 + 2 * x3 + rnorm(sampleSize, mean = 0, sd = sd3)
fit3 <- lm(y3 ~ x3)
res3 <- resid(fit3)
fitted3 <- fitted(fit3)
plot(
fitted1,
res1,
main = "Homoscedasticity",
xlab = "Fitted value",
ylab = "Residual")
abline(
h = 0,
lwd = 2,
col = "blue")
plot(
fitted2,
res2,
main = "Fan-Shaped Heteroscedasticity",
xlab = "Fitted value",
ylab = "Residual")
abline(
h = 0,
lwd = 2,
col = "blue")
plot(
fitted3,
res3,
main = "Bow-Tie-Shaped Heteroscedasticity",
xlab = "Fitted value",
ylab = "Residual")
abline(
h = 0,
lwd = 2,
col = "blue")
If there is heteroscedasticity, it may be necessary to transform the outcome variable to be more normally distributed. The spread-level plot provides a suggested power transformation to transform the outcome variable so that the spread of residuals becomes more uniform across the fitted values.
To examine whether residuals are normally distributed, we can examine quantile–quantile (QQ) plots and probability–probability (PP) plots. QQ plots are particularly useful for identifying deviations from normality in the tails of the distribution; PP plots are particularly useful for identifying deviations from normality in the center of the distribution. Researchers tend to be more concerned about the tails of the distribution, because extreme values tend to have a greater impact on inferences, so researchers tend to use QQ plots more often than PP plots. Various examples of QQ plots and deviations from normality are depicted in Figure 12.4. If the residuals are normally distributed, they will stay close to the diagonal reference line of the QQ and PP plots.
set.seed(52242)
sampleSize <- 10000
distribution_normal <- rnorm(
sampleSize,
mean = 0,
sd = 1)
distribution_bimodal <- c(
rnorm(
sampleSize/2,
mean = -2,
sd = 1),
rnorm(
sampleSize/2,
mean = 2,
sd = 1))
distribution_negativeSkew <- -rlnorm(
sampleSize,
meanlog = 0,
sdlog = 1)
distribution_positiveSkew <- rlnorm(
sampleSize,
meanlog = 0,
sdlog = 1)
distribution_lightTailed <- runif(
sampleSize,
min = -2,
max = 2)
distribution_heavyTailed <- rt(
sampleSize,
df = 2)
hist(
distribution_normal,
main = "Normal Distribution",
col = "#0099F8")
car::qqPlot(
distribution_normal,
main = "QQ Plot",
id = FALSE)
hist(
distribution_bimodal,
main = "Bimodal Distribution",
col = "#0099F8")
car::qqPlot(
distribution_bimodal,
main = "QQ Plot",
id = FALSE)
hist(
distribution_negativeSkew,
main = "Negatively Skewed Distribution",
col = "#0099F8")
car::qqPlot(
distribution_negativeSkew,
main = "QQ Plot",
id = FALSE)
hist(
distribution_positiveSkew,
main = "Positively Skewed Distribution",
col = "#0099F8")
car::qqPlot(
distribution_positiveSkew,
main = "QQ Plot",
id = FALSE)
hist(
distribution_lightTailed,
main = "Platykurtic (Light Tailed) Distribution",
col = "#0099F8")
car::qqPlot(
distribution_lightTailed,
main = "QQ Plot",
id = FALSE)
hist(
distribution_heavyTailed,
main = "Leptokurtic (Heavy Tailed) Distribution",
col = "#0099F8")
car::qqPlot(
distribution_heavyTailed,
main = "QQ Plot",
id = FALSE)
If the residuals are not normally distributed (i.e., they do not stay close to the diagonal reference line of the QQ and PP plots), it may be necessary to transform the outcome variable to be more normally distributed or to use a generalized linear model (GLM) that more closely matches the distribution of the outcome variable (e.g., Poisson, binomial, gamma).
When estimating a multiple regression model, it can be useful to evaluate how much variance in the outcome variable that the predictor variable(s) explain (i.e., account for). If the variables collectively explain only a small amount of variance, it suggest that the predictor variables have a small effect size and that other predictor variables will be necessary to account for the majority of variability in the outcome variable. There are two primary indices of how much how much variance in the outcome variable is explained by the predictor variable(s): the coefficient of determination (\(R^2\)) and adjusted \(R^2\) (\(R^2_{adj}\)), described below.
As noted in Section 10.6.6, The coefficient of determination (\(R^2\)) reflects the proportion of variance in the outcome (dependent) variable that is explained by the model predictions, as in Equation 10.25: \(R^2 = \frac{\text{variance explained in }Y}{\text{total variance in }Y}\). Various formulas for \(R^2\) are in Equation 10.19. Larger \(R^2\) values indicate greater accuracy. Multiple regression can be conceptualized with overlapping circles (similar to a venn diagram), where the non-overlapping portions of the circles reflect nonshared variance and the overlapping portions of the circles reflect shared variance, as in Figure 12.29.
One issue with \(R^2\) is that it increases as the number of predictors increases, which can lead to overfitting if using \(R^2\) as an index to compare models for purposes of selecting the “best-fitting” model. Consider the following example (adapted from Petersen (2025)) in which you have one predictor variable and one outcome variable, as shown in Table 12.1.
| y | x1 |
|---|---|
| 7 | 1 |
| 13 | 2 |
| 29 | 7 |
| 10 | 2 |
Using the data, the best fitting regression model is: \(y =\) 3.98 \(+\) 3.59 \(\cdot x_1\). In this example, the \(R^2\) is 0.98. The equation is not a perfect prediction, but with a single predictor variable, it captures the majority of the variance in the outcome.
Now consider the following example where you add a second predictor variable to the data above, as shown in Table 12.2.
| y | x1 | x2 |
|---|---|---|
| 7 | 1 | 3 |
| 13 | 2 | 5 |
| 29 | 7 | 1 |
| 10 | 2 | 2 |
With the second predictor variable, the best fitting regression model is: \(y =\) 0.00 + 4.00 \(\cdot x_1 +\) 1.00 \(\cdot x_2\). In this example, the \(R^2\) is 1.00. The equation with the second predictor variable provides a perfect prediction of the outcome.
Providing perfect prediction with the right set of predictor variables is the dream of multiple regression. So, using multiple regression, we often add predictor variables to incrementally improve prediction. Knowing how much variance would be accounted for by random chance follows Equation 12.3:
\[ E(R^2) = \frac{K}{n-1} \tag{12.3}\]
where \(E(R^2)\) is the expected value of \(R^2\) (the proportion of variance explained), \(K\) is the number of predictor variables, and \(n\) is the sample size. The formula demonstrates that the more predictor variables in the regression model, the more variance will be accounted for by chance. With many predictor variables and a small sample, you can account for a large share of the variance merely by chance.
As an example, consider that we have 13 predictor variables to predict fantasy performance for 43 players. Assume that, with 13 predictor variables, we explain 38% of the variance (\(R^2 = .38; r = .62\)). We explained a lot of the variance in the outcome, but it is important to consider how much variance could have been explained by random chance: \(E(R^2) = \frac{K}{n-1} = \frac{13}{43 - 1} = .31\). We expect to explain 31% of the variance, by chance, in the outcome. So, 82% of the variance explained was likely spurious (i.e., \(\frac{.31}{.38} = .82\)). As the sample size increases, the spuriousness decreases. To account for the number of predictor variables in the model, we can use a modified version of \(R^2\) called adjusted \(R^2\) (\(R^2_{adj}\)), described next.
Adjusted \(R^2\) (\(R^2_{adj}\)) accounts for the number of predictor variables in the model, based on how much would be expected to be accounted for by chance to penalize overfitting. Adjusted \(R^2\) (\(R^2_{adj}\)) reflects the proportion of variance in the outcome (dependent) variable that is explained by the model predictions over and above what would be expected to be accounted for by chance, given the number of predictor variables in the model. The formula for adjusted \(R^2\) (\(R^2_{adj}\)) is in Equation 12.4:
\[ R^2_{adj} = 1 - (1 - R^2) \frac{n - 1}{n - p - 1} \tag{12.4}\]
where \(p\) is the number of predictor variables in the model, and \(n\) is the sample size.
Statistical models applied to big data (e.g., data with many predictor variables) can overfit the data, which means that the statistical model accounts for error variance, which will not generalize to future samples. So, even though an overfitting statistical model appears to be accurate because it is accounting for more variance, it is not actually that accurate—it will predict new data less accurately than how accurately it accounts for the data with which the model was built. In the case of fantasy football analytics, this is especially relevant because there are hundreds if not thousands of variables we could consider for inclusion and many, many players when considering historical data.
Consider an example where you develop an algorithm to predict players’ fantasy performance based on 2024 data using hundreds of predictor variables. To some extent, these predictor variables will likely account for true variance (i.e., signal) and error variance (i.e., noise). If we were to apply the same algorithm based on the 2024 prediction model to 2025 data, the prediction model would likely predict less accurately than with 2024 data. The regression coefficients tend to become weaker when applied to new data, a phenomenon called shrinkage, which is described in Section 16.7.1. The regression coefficients in the [FILL IN]
In Figure 12.6, the blue line represents the true distribution of the data, and the red line is an overfitting model:
set.seed(52242)
sampleSize <- 200
quadraticX <- rnorm(sampleSize)
quadraticY <- quadraticX ^ 2 + rnorm(sampleSize)
quadraticData <- cbind(quadraticX, quadraticY) %>%
data.frame %>%
arrange(quadraticX)
quadraticModel <- lm(
quadraticY ~ quadraticX + I(quadraticX ^ 2),
data = quadraticData)
quadraticNewData <- data.frame(
quadraticX = seq(
from = min(quadraticData$quadraticX),
to = max(quadraticData$quadraticY),
length.out = sampleSize))
quadraticNewData$quadraticY <- predict(
quadraticModel,
newdata = quadraticNewData)
loessFit <- loess(
quadraticY ~ quadraticX,
data = quadraticData,
span = 0.01,
degree = 1)
loessNewData <- data.frame(
quadraticX = seq(
from = min(quadraticData$quadraticX),
to = max(quadraticData$quadraticY),
length.out = sampleSize))
quadraticNewData$loessY <- predict(
loessFit,
newdata = quadraticNewData)
plot(
x = quadraticData$quadraticX,
y = quadraticData$quadraticY,
xlab = "",
ylab = "")
lines(
quadraticNewData$quadraticY ~ quadraticNewData$quadraticX,
lwd = 2,
col = "blue")
lines(
quadraticNewData$loessY ~ quadraticNewData$quadraticX,
lwd = 2,
col = "red")Covariates are variables that you include in the statistical model to try to control for them so you can better isolate the unique contribution of the predictor variable(s) in relation to the outcome variable. Use of covariates examines the association between the predictor variable and the outcome variable when holding people’s level constant on the covariates. Inclusion of confounds as covariates allows potentially gaining a more accurate estimate of the causal effect of the predictor variable on the outcome variable. Ideally, you want to include any and all confounds as covariates. As described in Section 9.4.2.1, confounds are third variables that influence both the predictor variable and the outcome variable and explain their association. Covariates are potentially (but not necessarily) confounds. For instance, you might include the player’s age as a covariate in a model that examines whether a player’s 40-yard dash time at the NFL Combine predicts their fantasy points in their rookie year, but it may not be a confound.
Let’s say we want to use a number of variables to predict a wide receiver’s fantasy performance. We want to consider several predictors, including the player’s age, height, weight, and target share. We have only a few predictors and our sample size is large enough such that overfitting is not likely a concern.
On a weekly basis, target share is computed as the number of targets a player receives divided by the team’s total number of targets. It would be nice to compute it this way for seasonal data, as well; however, for calculating seasonal target share, the nflfastR (Carl & Baldwin, 2024) package currently sums up the player’s weekly target share. Thus, seasonal target share (unlike weekly target share)—as it is currently calculated—does not range from 0–1. For example, if a player has a weekly target share of 15%, that would be a seasonal (cross-week-summed) target share of 2.55 (\(0.15 \times 17 \; \text{games} = 2.55\).
Let’s first examine descriptive statistics of the predictor and outcome variables.
player_stats_seasonal %>%
dplyr::select(fantasyPoints, age, height, weight, target_share) %>%
dplyr::summarise(across(
everything(),
.fns = list(
n = ~ length(na.omit(.)),
missingness = ~ mean(is.na(.)) * 100,
M = ~ mean(., na.rm = TRUE),
SD = ~ sd(., na.rm = TRUE),
min = ~ min(., na.rm = TRUE),
max = ~ max(., na.rm = TRUE),
range = ~ max(., na.rm = TRUE) - min(., na.rm = TRUE),
IQR = ~ IQR(., na.rm = TRUE),
MAD = ~ mad(., na.rm = TRUE),
median = ~ median(., na.rm = TRUE),
pseudomedian = ~ DescTools::HodgesLehmann(., na.rm = TRUE),
mode = ~ petersenlab::Mode(., multipleModes = "mean"),
skewness = ~ psych::skew(., na.rm = TRUE),
kurtosis = ~ psych::kurtosi(., na.rm = TRUE)),
.names = "{.col}.{.fn}")) %>%
tidyr::pivot_longer(
cols = everything(),
names_to = c("variable","index"),
names_sep = "\\.") %>%
tidyr::pivot_wider(
names_from = index,
values_from = value)Let’s also examine the distributions of the variables using a density plot, as depicted in Figure 12.7.
ggplot2::ggplot(
data = player_stats_seasonal %>%
filter(position_group %in% c("WR")),
mapping = aes(
x = fantasyPoints)
) +
geom_histogram(
aes(y = after_stat(density)),
color = "#000000",
fill = "#0099F8"
) +
geom_density(
color = "#000000",
fill = "#F85700",
alpha = 0.6 # add transparency
) +
geom_rug() +
labs(
x = "Fantasy Points",
y = "Density",
title = "Density Plot of Fantasy Points with Histogram and Rug Plot"
) +
theme_classic() +
theme(axis.title.y = element_text(angle = 0, vjust = 0.5)) # horizontal y-axis title
ggplot2::ggplot(
data = player_stats_seasonal %>%
filter(position_group %in% c("WR")),
mapping = aes(
x = age)
) +
geom_histogram(
aes(y = after_stat(density)),
color = "#000000",
fill = "#0099F8"
) +
geom_density(
color = "#000000",
fill = "#F85700",
alpha = 0.6 # add transparency
) +
geom_rug() +
labs(
x = "Age (years)",
y = "Density",
title = "Density Plot of Player Age with Histogram and Rug Plot"
) +
theme_classic() +
theme(axis.title.y = element_text(angle = 0, vjust = 0.5)) # horizontal y-axis title
ggplot2::ggplot(
data = player_stats_seasonal %>%
filter(position_group %in% c("WR")),
mapping = aes(
x = height)
) +
geom_histogram(
aes(y = after_stat(density)),
color = "#000000",
fill = "#0099F8"
) +
geom_density(
color = "#000000",
fill = "#F85700",
alpha = 0.6 # add transparency
) +
geom_rug() +
labs(
x = "Height (inches)",
y = "Density",
title = "Density Plot of Player Height with Histogram and Rug Plot"
) +
theme_classic() +
theme(axis.title.y = element_text(angle = 0, vjust = 0.5)) # horizontal y-axis title
ggplot2::ggplot(
data = player_stats_seasonal %>%
filter(position_group %in% c("WR")),
mapping = aes(
x = weight)
) +
geom_histogram(
aes(y = after_stat(density)),
color = "#000000",
fill = "#0099F8"
) +
geom_density(
color = "#000000",
fill = "#F85700",
alpha = 0.6 # add transparency
) +
geom_rug() +
labs(
x = "Weight (pounds)",
y = "Density",
title = "Density Plot of Player Weight with Histogram and Rug Plot"
) +
theme_classic() +
theme(axis.title.y = element_text(angle = 0, vjust = 0.5)) # horizontal y-axis title
ggplot2::ggplot(
data = player_stats_seasonal %>%
filter(position_group %in% c("WR")),
mapping = aes(
x = target_share)
) +
geom_histogram(
aes(y = after_stat(density)),
color = "#000000",
fill = "#0099F8"
) +
geom_density(
color = "#000000",
fill = "#F85700",
alpha = 0.6 # add transparency
) +
geom_rug() +
labs(
x = "Target Share",
y = "Density",
title = "Density Plot of Target Share with Histogram and Rug Plot"
) +
theme_classic() +
theme(axis.title.y = element_text(angle = 0, vjust = 0.5)) # horizontal y-axis title
Then, let’s examine the bivariate association of each using a scatterplot to evaluate for any potential nonlinearity, as depicted in Figure 12.8.
ggplot2::ggplot(
data = player_stats_seasonal %>% filter(position %in% c("WR")),
aes(
x = age,
y = fantasyPoints)) +
geom_point(alpha = 0.05) +
geom_smooth(
method = "lm",
color = "black") +
geom_smooth() +
coord_cartesian(
ylim = c(0,NA),
expand = FALSE) +
labs(
x = "Player Age (Years)",
y = "Fantasy Points (Season)",
title = "Fantasy Points (Season) by Player Age",
subtitle = "(Among Wide Receivers)"
) +
theme_classic()
ggplot2::ggplot(
data = player_stats_seasonal %>% filter(position %in% c("WR")),
aes(
x = height,
y = fantasyPoints)) +
geom_point(alpha = 0.05) +
geom_smooth(
method = "lm",
color = "black") +
geom_smooth() +
coord_cartesian(
ylim = c(0,NA),
expand = FALSE) +
labs(
x = "Player Height (Inches)",
y = "Fantasy Points (Season)",
title = "Fantasy Points (Season) by Player Height",
subtitle = "(Among Wide Receivers)"
) +
theme_classic()
ggplot2::ggplot(
data = player_stats_seasonal %>% filter(position %in% c("WR")),
aes(
x = weight,
y = fantasyPoints)) +
geom_point(alpha = 0.05) +
geom_smooth(
method = "lm",
color = "black") +
geom_smooth() +
coord_cartesian(
ylim = c(0,NA),
expand = FALSE) +
labs(
x = "Player Weight (Pounds)",
y = "Fantasy Points (Season)",
title = "Fantasy Points (Season) by Player Weight",
subtitle = "(Among Wide Receivers)"
) +
theme_classic()
ggplot2::ggplot(
data = player_stats_seasonal %>% filter(position %in% c("WR")),
aes(
x = target_share,
y = fantasyPoints)) +
geom_point(alpha = 0.05) +
geom_smooth(
method = "lm",
color = "black") +
geom_smooth() +
coord_cartesian(
ylim = c(0,NA),
expand = FALSE) +
labs(
x = "Target Share",
y = "Fantasy Points (Season)",
title = "Fantasy Points (Season) by Target Share",
subtitle = "(Among Wide Receivers)"
) +
theme_classic()
There are some suggestions of potential nonlinearity, such as an inverted-U-shaped association between height and fantasy points, suggesting that there may an optimal range for height among Wide Receivers—being too short or too tall could be a disadvantage. In addition, target share shows a weakening association as target share increases. Thus, after evaluating the linear association between the predictors and outcome, we will also examine the possibility for curvilinear associations.
Now that we have examined descriptive statistics and bivariate associations, let’s first estimate a multiple regression model with only linear terms:
The model formula is in Equation 12.5:
\[ \text{fantasy points} = \beta_0 + \beta_1 \cdot \text{age} + \beta_2 \cdot \text{height} + \beta_3 \cdot \text{weight} + \beta_4 \cdot \text{target share} + \epsilon \tag{12.5}\]
Here are the model results:
Call:
lm(formula = fantasyPoints ~ age + height + weight + target_share,
data = player_stats_seasonal %>% filter(position %in% c("WR")),
na.action = "na.exclude")
Residuals:
Min 1Q Median 3Q Max
-739.80 -18.68 -10.50 10.74 292.78
Coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) -34.62684 23.85029 -1.452 0.146617
age 0.75193 0.22418 3.354 0.000803 ***
height 0.05816 0.42094 0.138 0.890107
weight 0.14716 0.06686 2.201 0.027794 *
target_share 743.14867 7.38684 100.604 < 2e-16 ***
---
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
Residual standard error: 45.71 on 4432 degrees of freedom
(952 observations deleted due to missingness)
Multiple R-squared: 0.7054, Adjusted R-squared: 0.7052
F-statistic: 2654 on 4 and 4432 DF, p-value: < 2.2e-16The only terms that are significantly associated with fantasy performance among Wide Receivers are weight and target share, both of which showed a positive association with fantasy points.
We can obtain the coefficient of determination (\(R^2\)) and adjusted \(R^2\) (\(R^2_{adj}\)) using the following code:
[1] 0.7054381[1] 0.7051723The model explained 71% of the variability in fantasy points (i.e., \(R^2 = .71\)).
The model formula with substituted values is in Equation 12.6:
\[ \begin{aligned} \text{fantasy points} = &\beta_0 + \beta_1 \cdot \text{age} + \beta_2 \cdot \text{height} + \beta_3 \cdot \text{weight} + \beta_4 \cdot \text{target share} + \epsilon \\ = &-34.63 + 0.75 \cdot \text{age} + 0.06 \cdot \text{height} + 0.15 \cdot \text{weight} + 743.15 \cdot \text{target share} + \epsilon \end{aligned} \tag{12.6}\]
If we want to obtain standardized regression coefficients, we can use the standardize_parameters() function of the effectsize package (Ben-Shachar et al., 2020, 2025).
# Standardization method: basic
Parameter | Std. Coef. | 95% CI
-----------------------------------------
(Intercept) | 0.00 | [ 0.00, 0.00]
age | 0.03 | [ 0.01, 0.04]
height | 1.63e-03 | [-0.02, 0.02]
weight | 0.03 | [ 0.00, 0.05]
target share | 0.83 | [ 0.82, 0.85]Or, we can standardize the outcome variable and each predictor variable using the scale() function:
Call:
lm(formula = scale(fantasyPoints) ~ scale(age) + scale(height) +
scale(weight) + scale(target_share), data = player_stats_seasonal %>%
filter(position %in% c("WR")), na.action = "na.exclude")
Residuals:
Min 1Q Median 3Q Max
-8.7953 -0.2221 -0.1249 0.1277 3.4808
Coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) -0.0003851 0.0081630 -0.047 0.962380
scale(age) 0.0281808 0.0084020 3.354 0.000803 ***
scale(height) 0.0016241 0.0117541 0.138 0.890107
scale(weight) 0.0259735 0.0118013 2.201 0.027794 *
scale(target_share) 0.8331762 0.0082817 100.604 < 2e-16 ***
---
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
Residual standard error: 0.5435 on 4432 degrees of freedom
(952 observations deleted due to missingness)
Multiple R-squared: 0.7054, Adjusted R-squared: 0.7052
F-statistic: 2654 on 4 and 4432 DF, p-value: < 2.2e-16# Standardization method: refit
Parameter | Std. Coef. | 95% CI
-----------------------------------------
(Intercept) | -8.91e-17 | [-0.02, 0.02]
age | 0.03 | [ 0.01, 0.04]
height | 1.63e-03 | [-0.02, 0.02]
weight | 0.03 | [ 0.00, 0.05]
target share | 0.83 | [ 0.82, 0.85]Target share has a large effect size. All of the other predictors have small effect size.
Now let’s consider whether any of the terms show curvilinear associations with fantasy points by adding quadratic terms:
The model formula is in Equation 12.7:
\[ \begin{aligned} \text{fantasy points} \; = \; &\beta_0 + \beta_1 \cdot \text{age} + \beta_2 \cdot \text{age}^2 + \beta_3 \cdot \text{height} + \beta_4 \cdot \text{height}^2 + \\ &\beta_5 \cdot \text{weight} + \beta_6 \cdot \text{weight}^2 + \beta_7 \cdot \text{target share} + \beta_8 \cdot \text{target share}^2 + \epsilon \end{aligned} \tag{12.7}\]
Here are the model results:
Call:
lm(formula = fantasyPoints ~ age + I(age^2) + height + I(height^2) +
weight + I(weight^2) + target_share + I(target_share^2),
data = player_stats_seasonal %>% filter(position %in% c("WR")),
na.action = "na.exclude")
Residuals:
Min 1Q Median 3Q Max
-276.575 -15.242 -3.012 6.187 310.190
Coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) 1.379e+02 4.258e+02 0.324 0.7462
age 3.698e+00 2.232e+00 1.657 0.0976 .
I(age^2) -5.963e-02 4.022e-02 -1.483 0.1382
height -3.861e+00 1.247e+01 -0.310 0.7568
I(height^2) 2.315e-02 8.601e-02 0.269 0.7878
weight -5.090e-01 7.525e-01 -0.676 0.4988
I(weight^2) 1.694e-03 1.868e-03 0.907 0.3646
target_share 1.121e+03 9.067e+00 123.605 <2e-16 ***
I(target_share^2) -9.512e+02 1.764e+01 -53.936 <2e-16 ***
---
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
Residual standard error: 35.52 on 4428 degrees of freedom
(952 observations deleted due to missingness)
Multiple R-squared: 0.8224, Adjusted R-squared: 0.822
F-statistic: 2563 on 8 and 4428 DF, p-value: < 2.2e-16Only the quadratic term for target share (not height or weight) shows a significant association.
Here are the standardized coefficients:
# Standardization method: basic
Parameter | Std. Coef. | 95% CI
--------------------------------------------
(Intercept) | 0.00 | [ 0.00, 0.00]
age | 0.14 | [-0.02, 0.30]
age^2 | -0.12 | [-0.28, 0.04]
height | -0.11 | [-0.79, 0.58]
height^2 | 0.09 | [-0.59, 0.78]
weight | -0.09 | [-0.35, 0.17]
weight^2 | 0.12 | [-0.14, 0.38]
target share | 1.26 | [ 1.24, 1.28]
target share^2 | -0.54 | [-0.56, -0.52]quadraticTermsRegressionModelStandardized <- lm(
scale(fantasyPoints) ~ scale(age) + scale(I(age^2)) + scale(height) + scale(I(height^2)) + scale(weight) + scale(I(weight^2)) + scale(target_share) + scale(I(target_share^2)),
data = player_stats_seasonal %>% filter(position %in% c("WR")),
na.action = "na.exclude"
)
summary(quadraticTermsRegressionModelStandardized)
Call:
lm(formula = scale(fantasyPoints) ~ scale(age) + scale(I(age^2)) +
scale(height) + scale(I(height^2)) + scale(weight) + scale(I(weight^2)) +
scale(target_share) + scale(I(target_share^2)), data = player_stats_seasonal %>%
filter(position %in% c("WR")), na.action = "na.exclude")
Residuals:
Min 1Q Median 3Q Max
-3.2881 -0.1812 -0.0358 0.0736 3.6878
Coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) -0.0009542 0.0063426 -0.150 0.8804
scale(age) 0.1386022 0.0836588 1.657 0.0976 .
scale(I(age^2)) -0.1246500 0.0840684 -1.483 0.1382
scale(height) -0.1078183 0.3480878 -0.310 0.7568
scale(I(height^2)) 0.0936578 0.3479758 0.269 0.7878
scale(weight) -0.0898412 0.1328264 -0.676 0.4988
scale(I(weight^2)) 0.1203022 0.1326650 0.907 0.3646
scale(target_share) 1.2565612 0.0101659 123.605 <2e-16 ***
scale(I(target_share^2)) -0.5428470 0.0100647 -53.936 <2e-16 ***
---
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
Residual standard error: 0.4222 on 4428 degrees of freedom
(952 observations deleted due to missingness)
Multiple R-squared: 0.8224, Adjusted R-squared: 0.822
F-statistic: 2563 on 8 and 4428 DF, p-value: < 2.2e-16# Standardization method: refit
Parameter | Std. Coef. | 95% CI
--------------------------------------------
(Intercept) | 0.10 | [ 0.08, 0.12]
age | 0.02 | [ 0.01, 0.04]
age^2 | -6.76e-03 | [-0.02, 0.00]
height | -0.01 | [-0.03, 0.00]
height^2 | 1.53e-03 | [-0.01, 0.01]
weight | 0.03 | [ 0.01, 0.05]
weight^2 | 4.48e-03 | [-0.01, 0.01]
target share | 1.07 | [ 1.05, 1.08]
target share^2 | -0.10 | [-0.10, -0.10]To visualize the regression coefficients, we can use the tidy() function of the broom package (Robinson et al., 2025), as in Figures 12.9 and 12.10.
ggplot(
quadraticTermsRegressionModel_tidy,
aes(
x = term,
y = estimate)) +
geom_point() +
geom_errorbar(
aes(
ymin = conf.low,
ymax = conf.high),
width = 0.2) +
coord_flip() + # flip axes for readability
labs(
title = "Regression Coefficients with 95% CI",
x = "Predictor",
y = "Coefficient Estimate") +
theme_minimal()
ggplot(
quadraticTermsRegressionModelStandardized_tidy,
aes(
x = term,
y = estimate)) +
geom_point() +
geom_errorbar(
aes(
ymin = conf.low,
ymax = conf.high),
width = 0.2) +
coord_flip() + # flip axes for readability
labs(
title = "Standardized Regression Coefficients with 95% CI",
x = "Predictor",
y = "Standardized Coefficient Estimate") +
theme_minimal()
If we wanted to visualize the shape of the model-implied association between target share and fantasy points, we could generate the model-implied predictions using the data range that we want to visualize.
newdata <- data.frame(
target_share = seq(
from = min(player_stats_seasonal$target_share[which(player_stats_seasonal$position == "WR")], na.rm = TRUE),
to = max(player_stats_seasonal$target_share[which(player_stats_seasonal$position == "WR")], na.rm = TRUE),
length.out = 1000
)
)
newdata$age <- mean(player_stats_seasonal$age[which(player_stats_seasonal$position == "WR")], na.rm = TRUE)
newdata$height <- mean(player_stats_seasonal$height[which(player_stats_seasonal$position == "WR")], na.rm = TRUE)
newdata$weight <- mean(player_stats_seasonal$weight[which(player_stats_seasonal$position == "WR")], na.rm = TRUE)We can depict the model-implied predictions of fantasy points as a function of target share, as shown in Figure 12.11.
ggplot2::ggplot(
data = newdata,
aes(
x = target_share,
y = fantasyPoints)) +
geom_smooth() +
coord_cartesian(
ylim = c(0,NA),
expand = FALSE) +
labs(
x = "Target Share",
y = "Fantasy Points (Season)",
title = "Fantasy Points (Season) by Target Share",
subtitle = "(Among Wide Receivers)"
) +
theme_classic()
We could also generate the model-implied prediction of fantasy points for any value of the predictor variables. For instance, here are the number of fantasy points expected for a Wide Receiver who is 23 years old, is 6’2” tall (72 inches), weighs 200 pounds, and has a (cross-week-summed) target share of 3 (which reflects an approximate target share of 17.6% percent per game; i.e., \(0.176 = \frac{3}{17 \; \text{games}}\)):
1
-5199.483 The player would be expected to score -5199 fantasy points.
We can calculate this by substituting in the regression coefficients and predictor values. Here is the computation:
age <- 23
height <- 72
weight <- 200
target_share <- 3
beta0 <- coef(quadraticTermsRegressionModel)[["(Intercept)"]]
beta1 <- coef(quadraticTermsRegressionModel)[["age"]]
beta2 <- coef(quadraticTermsRegressionModel)[["I(age^2)"]]
beta3 <- coef(quadraticTermsRegressionModel)[["height"]]
beta4 <- coef(quadraticTermsRegressionModel)[["I(height^2)"]]
beta5 <- coef(quadraticTermsRegressionModel)[["weight"]]
beta6 <- coef(quadraticTermsRegressionModel)[["I(weight^2)"]]
beta7 <- coef(quadraticTermsRegressionModel)[["target_share"]]
beta8 <- coef(quadraticTermsRegressionModel)[["I(target_share^2)"]]
predictedFantasyPoints <- beta0 + beta1*age + beta2*(age^2) + beta3*height + beta4*(height^2) + beta5*weight + beta6*(weight^2) + beta7*target_share + beta8*(target_share^2)
predictedFantasyPoints[1] -5199.483The model formula with substituted values is in Equation 12.8:
\[ \begin{aligned} \text{fantasy points} \; = \; &\beta_0 + \beta_1 \cdot \text{age} + \beta_2 \cdot \text{age}^2 + \beta_3 \cdot \text{height} + \beta_4 \cdot \text{height}^2 + \\ &\beta_5 \cdot \text{weight} + \beta_6 \cdot \text{weight}^2 + \beta_7 \cdot \text{target share} + \beta_8 \cdot \text{target share}^2 + \epsilon \\ = &137.86 + 3.70 \cdot 23 + -0.06 \cdot 23^2 + -3.86 \cdot 72 + 0.02 \cdot 72^2 + \\ &-0.51 \cdot 200 + 0.00 \cdot 200^2 + 1120.79 \cdot 3 + -951.24 \cdot 3^2 + \epsilon \\ = &-5199.48 \end{aligned} \tag{12.8}\]
The assumptions for multiple regression are described in Section 12.5. I describe ways to evaluate assumptions in Section 12.5.1.
As a reminder, here are four assumptions:
We evaluated the shape of the association between the predictor variables and the outcome variables using scatterplots. We accounted for potential curvilinearity in the associations with a quadratic term. Other ways to account for nonlinearity, in addition to polynomials, include transforming predictors, use of splines/piecewise regression, and generalized additive models.
To evaluate for potential nonlinearity in the associations, we can also evaluate residual plots (Figure 12.21), marginal model plots (Figure 12.12), added-variable plots (Figure 12.13), and component-plus-residual plots (Figure 12.14) from the car package (Fox et al., 2024; Fox & Weisberg, 2019). For evaluating linearity, we would expect minimal bend/curvature in the lines.
The marginal model plots (Figure 12.12), residual plots (Figure 12.21), and component-plus-residual plots (Figure 12.14) suggest that the nonlinearity of the association between target share and fantasy points may not be fully captured by the quadratic term. Thus, we may need to apply a different approach to handling the nonlinear association between target share and fantasy points.
One approach we can take is to transform the target_shares variable to be more normally distributed.
The histogram for the raw target_shares variable is in Figure 12.15.
The variable shows a strong positive skew. To address a strong positive skew, we can use a log transformation. The histogram of the log-transformed variable is in Figure 12.16.
Now we can re-fit the model with the log-transformed variable.
Call:
lm(formula = fantasyPoints ~ age + I(age^2) + height + I(height^2) +
weight + I(weight^2) + I(log(target_share + 1)), data = player_stats_seasonal %>%
filter(position %in% c("WR")), na.action = "na.exclude")
Residuals:
Min 1Q Median 3Q Max
-610.70 -15.60 -8.09 7.67 299.80
Coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) -1.130e+02 4.995e+02 -0.226 0.821
age 3.243e+00 2.619e+00 1.238 0.216
I(age^2) -4.782e-02 4.718e-02 -1.014 0.311
height 4.906e+00 1.462e+01 0.336 0.737
I(height^2) -3.416e-02 1.009e-01 -0.339 0.735
weight -1.168e+00 8.826e-01 -1.324 0.186
I(weight^2) 3.247e-03 2.191e-03 1.482 0.138
I(log(target_share + 1)) 8.974e+02 7.864e+00 114.108 <2e-16 ***
---
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
Residual standard error: 41.66 on 4429 degrees of freedom
(952 observations deleted due to missingness)
Multiple R-squared: 0.7555, Adjusted R-squared: 0.7551
F-statistic: 1955 on 7 and 4429 DF, p-value: < 2.2e-16Target share shows a more linear association with fantasy points after log-transforming it (albeit still not perfect), as depicted in Figures 12.17, 12.18, 12.19, and 12.20.
When creating a residual plot, the car package (Fox et al., 2024; Fox & Weisberg, 2019) also provides a test of the nonlinearity of each predictor.
Test stat Pr(>|Test stat|)
age 1.2532 0.2102
I(age^2) 1.0520 0.2928
height 0.2004 0.8412
I(height^2) 1.3782 0.1682
weight 0.7991 0.4243
I(weight^2) -0.4440 0.6571
I(log(target_share + 1)) -34.8271 <2e-16 ***
Tukey test -34.7456 <2e-16 ***
---
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
To evaluate homoscedasticity, we can evaluate a residual plot (Figure 12.21) and spread-level plot (Figure 12.22) from the car package (Fox et al., 2024; Fox & Weisberg, 2019). In a residual plot, you want a constant spread of residuals versus fitted values—you do not want the residuals to show a fan or cone shape.
Test stat Pr(>|Test stat|)
age 1.0848 0.27806
I(age^2) 1.9964 0.04595 *
height 0.5255 0.59927
I(height^2) 1.7659 0.07747 .
weight 0.8483 0.39634
I(weight^2) -0.7810 0.43485
target_share 1.5656 0.11751
I(target_share^2) -15.0992 < 2e-16 ***
Tukey test 20.2150 < 2e-16 ***
---
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
In a spread-level plot, you want a flat (zero) slope—you do not want a positive or negative slope.
Suggested power transformation: 0.4647539 
In this example, the residuals appear to increase as a function of the fitted values. To handle this, we may need to transform the outcome variable to be more normally distributed.
The histogram for raw fantasy points is in Figure 13.34.
We can apply a Yeo-Johnson transformation to the outcome variable to generate a more normally distributed variable. A Yeo-Johnson transformation estimates the optimal transformation to make the variable more normally distributed. Let’s use a Yeo-Johnson transformation of fantasy points:
Created from 41156 samples and 1 variables
Pre-processing:
- ignored (0)
- Yeo-Johnson transformation (1)
Lambda estimates for Yeo-Johnson transformation:
0.18The histogram of the transformed variable is in Figure 12.24.
Now we can refit the model.
Call:
lm(formula = fantasyPoints_transformed ~ age + height + weight +
I(log(target_share + 1)), data = player_stats_seasonal %>%
filter(position %in% c("WR")), na.action = "na.exclude")
Residuals:
Min 1Q Median 3Q Max
-18.3306 -0.6618 0.2290 0.8845 6.7486
Coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) 2.373486 0.821447 2.889 0.00388 **
age 0.033732 0.007723 4.367 1.29e-05 ***
height -0.009173 0.014496 -0.633 0.52690
weight 0.003643 0.002302 1.582 0.11366
I(log(target_share + 1)) 27.866921 0.296506 93.984 < 2e-16 ***
---
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
Residual standard error: 1.574 on 4432 degrees of freedom
(952 observations deleted due to missingness)
Multiple R-squared: 0.6767, Adjusted R-squared: 0.6764
F-statistic: 2319 on 4 and 4432 DF, p-value: < 2.2e-16The residual plot is in Figure 12.25.
Test stat Pr(>|Test stat|)
age 0.0663 0.9472
height -0.3612 0.7180
weight -1.5995 0.1098
I(log(target_share + 1)) -48.2540 <2e-16 ***
Tukey test -48.8390 <2e-16 ***
---
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
The spread-level plot is in Figure 12.26.
Suggested power transformation: 1.890813 
The residuals show more constant variance after transforming the outcome variable.
We can examine whether residuals are normally distributed using quantile–quantile (QQ) plots and probability–probability (PP) plots, as in Figures 12.27 and 12.28. If the residuals are normally distributed, they should stay close to the diagonal reference line.
[1] 1338 1843
Multicollinearity occurs when two or more predictor variables in a regression model are highly correlated. The problem of having multiple predictor variables that are highly correlated is that it makes it challenging to estimate the regression coefficients accurately.
Multicollinearity in multiple regression is depicted conceptually in Figure 12.29.
Consider the following example adapted from Petersen (2025) where you have two predictor variables and one outcome variable, as shown in Table 12.3.
| y | x1 | x2 |
|---|---|---|
| 9 | 2.0 | 4 |
| 11 | 3.0 | 6 |
| 17 | 4.0 | 8 |
| 3 | 1.0 | 2 |
| 21 | 5.0 | 10 |
| 13 | 3.5 | 7 |
The second predictor variable is not very good—it is exactly twice the value of the first predictor variable; thus, the two predictor variables are perfectly correlated (i.e., \(r = 1.0\)). This means that there are different prediction equation possibilities that are equally good—see Equations in Equation 12.9:
\[ \begin{aligned} 2x_2 &= y \\ 0x_1 + 2x_2 &= y \\ 4x_1 &= y \\ 4x_1 + 0x_2 &= y \\ 2x_1 + 1x_2 &= y \\ 5x_1 - 0.5x_2 &= y \\ ... &= y \end{aligned} \tag{12.9}\]
Then, what are the regression coefficients? We do not know what are the correct regression coefficients because each of the possibilities fits the data equally well. Thus, when estimating the regression model, we could obtain arbitrary estimates of the regression coefficients with an enormous standard error around each estimate. In general, multicollinearity increases the uncertainty (i.e., standard errors and confidence intervals) around the parameter estimates. Any predictor variables that have a correlation above ~ \(r = .30\) with each other could have an impact on the confidence interval of the regression coefficient. As the correlations among the predictor variables increase, the chance of getting an arbitrary answer increases, sometimes called “bouncing betas.” So, it is important to examine a correlation matrix of the predictor variables before putting them in the same regression model. You can also examine indices such as variance inflation factor (VIF), where a value greater than 5 or 10 indicates multicollinearity.
Here are the VIFs from our earlier model:
age height weight
1.019535 2.095026 2.111745
I(log(target_share + 1))
1.030896 To address multicollinearity, you can drop a redundant predictor or you can also use principal component analysis or factor analysis of the predictors to reduce the predictors down to a smaller number of meaningful predictors. For a meaningful answer regarding predictors in a regression framework that is precise and confident, you need a low level of intercorrelation among predictors, unless you have a very large sample size. However, if you are merely interested in prediction—and are not interested in interpreting the regression coefficients of individual predictors—multicollinearity poses less of a problem. For instance, machine learning cares more about achieving the greatest predictive accuracy possible and cares less about explaining which predictors are causally related to the outcome. So, multicollinearity is less of a concern for machine learning approaches.
An important consideration in multiple regression is how missing data are handled. Multiple regression in R using the lm() function applies listwise deletion. Listwise deletion removes any row (in the data file) from analysis that has a missing value on the outcome variable or any of the predictor variables. Removing all rows from analysis that have any missingness in the model variables can be a problem because missingness is often not completely at random—missingness often occurs systematically (i.e., for a reason). For instance, participants may be less likely to have data for all variables if they are from a lower socioeconomic status background and do not have the time to participate in all study procedures. Thus, applying listwise deletion, we might systematically exclude participants from lower socioeconomic status backgrounds (or other groups), which could lead to less generalizable inferences.
It is thus important to consider approaches to handle missingness. Various approaches to handle missingness include pairwise deletion (aka available-case analysis), multiple imputation, and full information maximum likelihood (FIML).
We can estimate a regression model that uses pairwise deletion using the lavaan package (Rosseel, 2012; Rosseel et al., 2024).
player_stats_seasonal$target_share_log <- log(player_stats_seasonal$target_share + 1)
modelData <- player_stats_seasonal %>%
filter(position %in% c("WR")) %>%
select(fantasyPoints_transformed, age, height, weight, target_share_log)
numObs <- sum(complete.cases(modelData))
varMeans <- colMeans(modelData, na.rm = TRUE)
varCovariances <- cov(modelData, use = "pairwise.complete.obs")
pairwiseRegression_syntax <- '
fantasyPoints_transformed ~ age + height + weight + target_share_log
fantasyPoints_transformed ~~ fantasyPoints_transformed
fantasyPoints_transformed ~ 1
'
pairwiseRegression_fit <- lavaan::lavaan(
pairwiseRegression_syntax,
sample.mean = varMeans,
sample.cov = varCovariances,
sample.nobs = numObs
)
summary(
pairwiseRegression_fit,
standardized = TRUE,
rsquare = TRUE)lavaan 0.6-19 ended normally after 1 iteration
Estimator ML
Optimization method NLMINB
Number of model parameters 6
Number of observations 4437
Model Test User Model:
Test statistic 0.000
Degrees of freedom 0
Parameter Estimates:
Standard errors Standard
Information Expected
Information saturated (h1) model Structured
Regressions:
Estimate Std.Err z-value P(>|z|) Std.lv
fantasyPoints_transformed ~
age 0.055 0.008 7.305 0.000 0.055
height 0.008 0.014 0.574 0.566 0.008
weight 0.004 0.002 1.645 0.100 0.004
target_shar_lg 27.725 0.295 94.041 0.000 27.725
Std.all
0.063
0.007
0.020
0.811
Intercepts:
Estimate Std.Err z-value P(>|z|) Std.lv Std.all
.fntsyPnts_trns 0.543 0.818 0.664 0.507 0.543 0.196
Variances:
Estimate Std.Err z-value P(>|z|) Std.lv Std.all
.fntsyPnts_trns 2.450 0.052 47.101 0.000 2.450 0.320
R-Square:
Estimate
fntsyPnts_trns 0.680We can multiply impute data using the mice package (van Buuren & Groothuis-Oudshoorn, 2011, 2024).
numImputations <- 5
dataToImpute <- player_stats_seasonal %>%
filter(position %in% c("WR")) %>%
select(player_id, position, where(is.numeric)) %>%
select(
player_id:games, carries:wopr, fantasy_points, fantasy_points_ppr,
rush_40_yds, rec_40_yds, fumbles, two_pts, return_yds,
rush_100_yds:draftround, height:target_share_log) %>%
select(-c(fantasy_points_ppr, ageCentered20, target_share_log)) # drop collinear variables
predictors <- c("targets","receiving_yards","receiving_air_yards","receiving_yards_after_catch","receiving_first_downs","racr")
dataToImpute$player_id_integer <- as.integer(as.factor(dataToImpute$player_id))
varsToImpute <- c("age","height","weight","target_share")
Y <- varsToImputeNow, let’s specify the imputation method—we use the two-level predictive mean matching (2l.pmm) method from the miceadds package (Robitzsch et al., 2024) to account for the nonindependent data (owing to multiple seasons per player):
Now, let’s specify the prediction matrix. A predictor matrix is a matrix of values, where:
The values are:
0
-2
1
2
3
4
Now, let’s run the imputation:
Below are some imputation diagnostics. Trace plots are in Figure 12.30.
A density plot is in Figure 12.31.
The imputated data does not match well the distribution of the observed data. Thus, it may be necessary to select a different imputation method for more accurate imputation.
Now, let’s do some post-processing:
Now let’s estimate multiple regression with the multiply imputed data:
We can estimate a regression model that uses full information maximum likelihood using the lavaan package (Rosseel, 2012; Rosseel et al., 2024).
fimlRegression_syntax <- '
fantasyPoints_transformed ~ age + height + weight + target_share_log
fantasyPoints_transformed ~~ fantasyPoints_transformed
fantasyPoints_transformed ~ 1
'
fimlRegression_fit <- lavaan::sem(
fimlRegression_syntax,
data = player_stats_seasonal %>% filter(position %in% c("WR")),
missing = "ML",
fixed.x = FALSE
)
summary(
fimlRegression_fit,
standardized = TRUE,
rsquare = TRUE)lavaan 0.6-19 ended normally after 58 iterations
Estimator ML
Optimization method NLMINB
Number of model parameters 20
Number of observations 5389
Number of missing patterns 2
Model Test User Model:
Test statistic 0.000
Degrees of freedom 0
Parameter Estimates:
Standard errors Standard
Information Observed
Observed information based on Hessian
Regressions:
Estimate Std.Err z-value P(>|z|) Std.lv
fantasyPoints_transformed ~
age 0.040 0.007 5.443 0.000 0.040
height -0.003 0.014 -0.212 0.832 -0.003
weight 0.004 0.002 1.741 0.082 0.004
target_shar_lg 27.770 0.285 97.531 0.000 27.770
Std.all
0.046
-0.003
0.021
0.813
Covariances:
Estimate Std.Err z-value P(>|z|) Std.lv Std.all
age ~~
height -0.297 0.101 -2.945 0.003 -0.297 -0.040
weight -0.155 0.637 -0.243 0.808 -0.155 -0.003
target_shar_lg 0.037 0.004 10.100 0.000 0.037 0.145
height ~~
weight 24.979 0.584 42.755 0.000 24.979 0.716
target_shar_lg 0.016 0.003 5.808 0.000 0.016 0.082
weight ~~
target_shar_lg 0.152 0.017 8.931 0.000 0.152 0.127
Intercepts:
Estimate Std.Err z-value P(>|z|) Std.lv Std.all
.fntsyPnts_trns 1.704 0.796 2.142 0.032 1.704 0.616
age 26.254 0.043 611.427 0.000 26.254 8.329
height 72.606 0.032 2269.504 0.000 72.606 30.916
weight 200.480 0.202 991.395 0.000 200.480 13.505
target_shar_lg 0.081 0.001 70.753 0.000 0.081 0.997
Variances:
Estimate Std.Err z-value P(>|z|) Std.lv Std.all
.fntsyPnts_trns 2.467 0.052 47.691 0.000 2.467 0.322
age 9.936 0.191 51.909 0.000 9.936 1.000
height 5.516 0.106 51.908 0.000 5.516 1.000
weight 220.372 4.245 51.908 0.000 220.372 1.000
target_shar_lg 0.007 0.000 49.109 0.000 0.007 1.000
R-Square:
Estimate
fntsyPnts_trns 0.678Please note that the \(p\)-value for regression coefficients assumes that the observations are independent—in particular, that the residuals are not correlated. However, the observations are not independent in the player_stats_seasonal dataframe used above, because the same player has multiple rows—one row corresponding to each season they played. This non-independence violates the traditional assumptions of the significance of regression coefficients. We could address this assumption by analyzing only one season from each player or by estimating the significance of the regression coefficients using cluster-robust standard errors. For simplicity in the models above, we present results above from the whole dataframe. In Chapter 13, we discuss mixed model approaches that handle repeated measures and other data that violate assumptions of non-independence. Below, we demonstrate how to account for non-independence of observations using cluster-robust standard errors with the rms package (Harrell, Jr., 2025).
player_stats_seasonal_subset <- player_stats_seasonal %>%
filter(!is.na(player_id)) %>%
filter(position %in% c("WR"))
rms::robcov(rms::ols(
fantasyPoints_transformed ~ age + height + weight + I(log(target_share + 1)),
data = player_stats_seasonal_subset,
x = TRUE,
y = TRUE),
cluster = player_stats_seasonal_subset$player_id) #account for nested data within playerFrequencies of Missing Values Due to Each Variable
fantasyPoints_transformed age height
0 0 0
weight target_share
0 952
Linear Regression Model
rms::ols(formula = fantasyPoints_transformed ~ age + height +
weight + I(log(target_share + 1)), data = player_stats_seasonal_subset,
x = TRUE, y = TRUE)
Model Likelihood Discrimination
Ratio Test Indexes
Obs 4437 LR chi2 5009.98 R2 0.677
sigma 1.5744 d.f. 4 R2 adj 0.676
d.f. 4432 Pr(> chi2) 0.0000 g 2.428
Cluster onplayer_stats_seasonal_subset$player_id
Clusters 1292
Residuals
Min 1Q Median 3Q Max
-18.3306 -0.6618 0.2290 0.8845 6.7486
Coef S.E. t Pr(>|t|)
Intercept 2.3735 1.0482 2.26 0.0236
age 0.0337 0.0089 3.78 0.0002
height -0.0092 0.0183 -0.50 0.6171
weight 0.0036 0.0027 1.34 0.1789
target_share 27.8669 1.0949 25.45 <0.0001 As with correlation, multiple regression can be strongly impacted by outliers.
To address outliers, there are various approaches to robust regression. One approach is to use an MM-type estimator, such as is used in the lmrob() and glmrob() functions of the robustbase package (Maechler et al., 2024; Todorov & Filzmoser, 2009).
Call:
lmrob(formula = fantasyPoints_transformed ~ age + height + weight + I(log(target_share +
1)), data = player_stats_seasonal %>% filter(position %in% c("WR")))
\--> method = "MM"
Residuals:
Min 1Q Median 3Q Max
-20.8013 -0.7763 0.1021 0.7743 7.0129
Coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) 3.9267186 0.6750051 5.817 6.4e-09 ***
age 0.0033425 0.0056495 0.592 0.554
height -0.0146743 0.0120894 -1.214 0.225
weight 0.0006476 0.0018724 0.346 0.729
I(log(target_share + 1)) 31.7037140 0.3244958 97.701 < 2e-16 ***
---
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
Robust residual standard error: 1.053
(952 observations deleted due to missingness)
Multiple R-squared: 0.8088, Adjusted R-squared: 0.8086
Convergence in 15 IRWLS iterations
Robustness weights:
56 observations c(134,175,221,260,277,343,368,480,481,705,706,1059,1094,1114,1130,1131,1186,1314,1315,1350,1425,1437,1486,1513,1549,1699,2172,2188,2203,2228,2264,2437,2577,2636,2671,2745,2779,2787,2918,2946,3251,3260,3428,3508,3551,3669,3697,3788,3905,3920,3921,4042,4061,4158,4242,4372)
are outliers with |weight| = 0 ( < 2.3e-05);
332 weights are ~= 1. The remaining 4049 ones are summarized as
Min. 1st Qu. Median Mean 3rd Qu. Max.
0.0004601 0.8472000 0.9408000 0.8754000 0.9837000 0.9990000
Algorithmic parameters:
tuning.chi bb tuning.psi refine.tol
1.548e+00 5.000e-01 4.685e+00 1.000e-07
rel.tol scale.tol solve.tol zero.tol
1.000e-07 1.000e-10 1.000e-07 1.000e-10
eps.outlier eps.x warn.limit.reject warn.limit.meanrw
2.254e-05 4.820e-10 5.000e-01 5.000e-01
nResample max.it groups n.group best.r.s
500 50 5 400 2
k.fast.s k.max maxit.scale trace.lev mts
1 200 200 0 1000
compute.rd fast.s.large.n
0 2000
psi subsampling cov
"bisquare" "nonsingular" ".vcov.avar1"
compute.outlier.stats
"SM"
seed : int(0) When examining moderation in multiple regression, several steps are important:
First, we mean-center the predictors. In this case, we center the predictors around the mean of height and weight for Wide Receivers:
player_stats_seasonal$height_centered <- player_stats_seasonal$height - mean(player_stats_seasonal$height[which(player_stats_seasonal$position == "WR")], na.rm = TRUE)
player_stats_seasonal$weight_centered <- player_stats_seasonal$weight - mean(player_stats_seasonal$weight[which(player_stats_seasonal$position == "WR")], na.rm = TRUE)Then, we compute the interaction term as the multiplication of the two centered predictors:
Then, we fit the moderated multiple regression model:
Call:
lm(formula = fantasyPoints_transformed ~ height_centered + weight_centered +
height_centered:weight_centered, data = player_stats_seasonal %>%
filter(position %in% c("WR")), na.action = "na.exclude")
Residuals:
Min 1Q Median 3Q Max
-11.1162 -2.0928 0.3443 2.2965 5.5387
Coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) 5.6129049 0.0445328 126.040 < 2e-16 ***
height_centered -0.0289282 0.0229691 -1.259 0.2079
weight_centered 0.0259022 0.0036211 7.153 9.62e-13 ***
height_centered:weight_centered -0.0017483 0.0009675 -1.807 0.0708 .
---
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
Residual standard error: 2.746 on 5385 degrees of freedom
Multiple R-squared: 0.01561, Adjusted R-squared: 0.01506
F-statistic: 28.46 on 3 and 5385 DF, p-value: < 2.2e-16This model is equivalent to the model that includes the interaction term explicitly:
Now, we can visualize the interaction to understand it. We create an interaction plot (Figure 12.32) and Johnson-Neyman plot (Figure 12.33) using the interactions package (Long, 2024).
JOHNSON-NEYMAN INTERVAL
When weight_centered is INSIDE the interval [16.16, 136.16], the slope of
height_centered is p < .05.
Note: The range of observed values of weight_centered is [-47.48, 64.52]
Here is a simple slopes analysis:
SIMPLE SLOPES ANALYSIS
Slope of height_centered when weight_centered = -1.484626e+01 (- 1 SD):
Est. S.E. t val. p
------- ------ -------- ------
-0.00 0.03 -0.12 0.91
Slope of height_centered when weight_centered = -1.002064e-16 (Mean):
Est. S.E. t val. p
------- ------ -------- ------
-0.03 0.02 -1.26 0.21
Slope of height_centered when weight_centered = 1.484626e+01 (+ 1 SD):
Est. S.E. t val. p
------- ------ -------- ------
-0.05 0.03 -1.93 0.05A mediation model takes the following general form in the lavaan package (Rosseel, 2012; Rosseel et al., 2024).
Let’s substitute in our predictor, outcome, and hypothesized mediator. In this case, we predict that receiving touchdowns partially accounts for the association between Wide Receiver’s target share and their fantasy points. This is a silly example because fantasy points are derived, in part, from touchdowns, so of course touchdowns will partially account for almost any effect on Wide Receivers’ fantasy points. This example is merely for demonstrating the process of developing and examining a mediation model.
To get a robust estimate of the indirect effect, we obtain bootstrapped estimates from 1,000 bootstrap draws. Typically, we would obtain bootstrapped estimates from 10,000 bootstrap draws, but this example uses only 1,000 bootstrap draws for a shorter runtime.
mediationFit <- lavaan::sem(
mediationModel,
data = player_stats_seasonal %>% filter(position %in% c("WR")),
se = "bootstrap",
bootstrap = 1000, # generally use 10,000 bootstrap draws; this example uses 1,000 for speed
parallel = "multicore", # parallelization for speed: use "multicore" for Mac/Linux; "snow" for PC
iseed = 52242, # for reproducibility
missing = "ML",
estimator = "ML",
fixed.x = FALSE)Here are the model results:
lavaan 0.6-19 ended normally after 16 iterations
Estimator ML
Optimization method NLMINB
Number of model parameters 9
Number of observations 5389
Number of missing patterns 2
Model Test User Model:
Test statistic 0.000
Degrees of freedom 0
Model Test Baseline Model:
Test statistic 9274.292
Degrees of freedom 3
P-value 0.000
User Model versus Baseline Model:
Comparative Fit Index (CFI) 1.000
Tucker-Lewis Index (TLI) 1.000
Robust Comparative Fit Index (CFI) 1.000
Robust Tucker-Lewis Index (TLI) 1.000
Loglikelihood and Information Criteria:
Loglikelihood user model (H0) -17772.527
Loglikelihood unrestricted model (H1) -17772.527
Akaike (AIC) 35563.053
Bayesian (BIC) 35622.382
Sample-size adjusted Bayesian (SABIC) 35593.783
Root Mean Square Error of Approximation:
RMSEA 0.000
90 Percent confidence interval - lower 0.000
90 Percent confidence interval - upper 0.000
P-value H_0: RMSEA <= 0.050 NA
P-value H_0: RMSEA >= 0.080 NA
Robust RMSEA 0.000
90 Percent confidence interval - lower 0.000
90 Percent confidence interval - upper 0.000
P-value H_0: Robust RMSEA <= 0.050 NA
P-value H_0: Robust RMSEA >= 0.080 NA
Standardized Root Mean Square Residual:
SRMR 0.000
Parameter Estimates:
Standard errors Bootstrap
Number of requested bootstrap draws 1000
Number of successful bootstrap draws 1000
Regressions:
Estimate Std.Err z-value P(>|z|) Std.lv
fantasyPoints_transformed ~
trgt_sh (drct) 14.213 1.533 9.271 0.000 14.213
receiving_tds ~
trgt_sh (a) 21.853 1.309 16.696 0.000 21.853
fantasyPoints_transformed ~
rcvng_t (b) 0.401 0.035 11.393 0.000 0.401
Std.all
0.485
0.701
0.426
Intercepts:
Estimate Std.Err z-value P(>|z|) Std.lv Std.all
.fntsyPnts_trns 3.420 0.059 58.149 0.000 3.420 1.236
.receiving_tds 0.335 0.101 3.303 0.001 0.335 0.114
target_share 0.088 0.001 64.770 0.000 0.088 0.929
Variances:
Estimate Std.Err z-value P(>|z|) Std.lv Std.all
.fntsyPnts_trns 2.247 0.103 21.744 0.000 2.247 0.294
.receiving_tds 4.399 0.287 15.336 0.000 4.399 0.508
target_share 0.009 0.001 15.897 0.000 0.009 1.000
R-Square:
Estimate
fntsyPnts_trns 0.706
receiving_tds 0.492
Defined Parameters:
Estimate Std.Err z-value P(>|z|) Std.lv Std.all
indirect 8.760 0.328 26.702 0.000 8.760 0.299
total 22.973 1.293 17.773 0.000 22.973 0.784We can also estimate a model with multiple hypothesized mediator:
multipleMediatorModel <- '
# direct effect (cPrime)
fantasyPoints_transformed ~ direct*target_share
# mediator
receiving_tds ~ a1*target_share
receiving_yards ~ a2*target_share
fantasyPoints_transformed ~ b1*receiving_tds + b2*receiving_yards
# indirect effect = a*b
indirect1 := a1*b1
indirect2 := a2*b2
indirectTotal := indirect1 + indirect2
# total effect (c)
total := abs(direct) + abs(indirectTotal)
'multipleMediatorFit <- lavaan::sem(
multipleMediatorModel,
data = player_stats_seasonal %>% filter(position %in% c("WR")),
se = "bootstrap",
bootstrap = 1000, # generally use 10,000 bootstrap draws; this example uses 1,000 for speed
parallel = "multicore", # parallelization for speed: use "multicore" for Mac/Linux; "snow" for PC
iseed = 52242, # for reproducibility
missing = "ML",
estimator = "ML",
fixed.x = FALSE)Here are the model results:
lavaan 0.6-19 ended normally after 33 iterations
Estimator ML
Optimization method NLMINB
Number of model parameters 13
Number of observations 5389
Number of missing patterns 2
Model Test User Model:
Test statistic 3190.289
Degrees of freedom 1
P-value (Chi-square) 0.000
Model Test Baseline Model:
Test statistic 20796.992
Degrees of freedom 6
P-value 0.000
User Model versus Baseline Model:
Comparative Fit Index (CFI) 0.847
Tucker-Lewis Index (TLI) 0.080
Robust Comparative Fit Index (CFI) 0.842
Robust Tucker-Lewis Index (TLI) 0.049
Loglikelihood and Information Criteria:
Loglikelihood user model (H0) -53416.617
Loglikelihood unrestricted model (H1) -51821.473
Akaike (AIC) 106859.235
Bayesian (BIC) 106944.932
Sample-size adjusted Bayesian (SABIC) 106903.623
Root Mean Square Error of Approximation:
RMSEA 0.769
90 Percent confidence interval - lower 0.747
90 Percent confidence interval - upper 0.792
P-value H_0: RMSEA <= 0.050 0.000
P-value H_0: RMSEA >= 0.080 1.000
Robust RMSEA 0.804
90 Percent confidence interval - lower 0.778
90 Percent confidence interval - upper 0.830
P-value H_0: Robust RMSEA <= 0.050 0.000
P-value H_0: Robust RMSEA >= 0.080 1.000
Standardized Root Mean Square Residual:
SRMR 0.080
Parameter Estimates:
Standard errors Bootstrap
Number of requested bootstrap draws 1000
Number of successful bootstrap draws 1000
Regressions:
Estimate Std.Err z-value P(>|z|) Std.lv
fantasyPoints_transformed ~
trgt_sh (drct) 4.121 0.792 5.204 0.000 4.121
receiving_tds ~
trgt_sh (a1) 22.440 1.281 17.514 0.000 22.440
receiving_yards ~
trgt_sh (a2) 3472.583 196.560 17.667 0.000 3472.583
fantasyPoints_transformed ~
rcvng_t (b1) 0.039 0.011 3.500 0.000 0.039
rcvng_y (b2) 0.005 0.000 27.240 0.000 0.005
Std.all
0.143
0.728
0.848
0.042
0.732
Intercepts:
Estimate Std.Err z-value P(>|z|) Std.lv Std.all
.fntsyPnts_trns 3.190 0.030 104.646 0.000 3.190 1.161
.receiving_tds 0.282 0.099 2.857 0.004 0.282 0.096
.receiving_yrds 70.408 15.353 4.586 0.000 70.408 0.180
target_share 0.088 0.001 65.149 0.000 0.088 0.920
Variances:
Estimate Std.Err z-value P(>|z|) Std.lv Std.all
.fntsyPnts_trns 1.646 0.036 45.284 0.000 1.646 0.218
.receiving_tds 4.062 0.261 15.542 0.000 4.062 0.469
.receiving_yrds 42850.451 5566.673 7.698 0.000 42850.451 0.280
target_share 0.009 0.001 15.159 0.000 0.009 1.000
R-Square:
Estimate
fntsyPnts_trns 0.782
receiving_tds 0.531
receiving_yrds 0.720
Defined Parameters:
Estimate Std.Err z-value P(>|z|) Std.lv Std.all
indirect1 0.871 0.249 3.496 0.000 0.871 0.030
indirect2 17.858 0.595 30.010 0.000 17.858 0.621
indirectTotal 18.729 0.581 32.247 0.000 18.729 0.651
total 22.849 1.278 17.873 0.000 22.849 0.794 Family: gaussian
Links: mu = identity; sigma = identity
Formula: fantasyPoints_transformed ~ age + height + weight + I(log(target_share + 1))
Data: player_stats_seasonal %>% filter(position %in% c(" (Number of observations: 4437)
Draws: 4 chains, each with iter = 2000; warmup = 1000; thin = 1;
total post-warmup draws = 4000
Regression Coefficients:
Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS Tail_ESS
Intercept 2.37 0.82 0.76 3.95 1.00 3779 3268
age 0.03 0.01 0.02 0.05 1.00 4499 2979
height -0.01 0.01 -0.04 0.02 1.00 3472 2597
weight 0.00 0.00 -0.00 0.01 1.00 3686 3511
Ilogtarget_shareP1 27.86 0.30 27.26 28.45 1.00 3745 2698
Further Distributional Parameters:
Estimate Est.Error l-95% CI u-95% CI Rhat Bulk_ESS Tail_ESS
sigma 1.57 0.02 1.54 1.61 1.00 3590 2406
Draws were sampled using sampling(NUTS). For each parameter, Bulk_ESS
and Tail_ESS are effective sample size measures, and Rhat is the potential
scale reduction factor on split chains (at convergence, Rhat = 1).Multiple regression allows examining the association between multiple predictor variables and one outcome variable. Inclusion of multiple predictors in the model allows for potentially greater predictive accuracy and identification of the extent to which each variable uniquely contributes to the outcome variable. As with correlation, an association does not imply causation. However, identifying associations is important because associations are a necessary (but insufficient) condition for causality. When developing a multiple regression model, there are various assumptions that are important to evaluate. In addition, it is important to pay attention for potential multicollinearity—it may become difficult to detect a given predictor variable as statistically significant due to the greater uncertainty around the parameter estimates.
R version 4.5.1 (2025-06-13)
Platform: x86_64-pc-linux-gnu
Running under: Ubuntu 24.04.2 LTS
Matrix products: default
BLAS: /usr/lib/x86_64-linux-gnu/openblas-pthread/libblas.so.3
LAPACK: /usr/lib/x86_64-linux-gnu/openblas-pthread/libopenblasp-r0.3.26.so; LAPACK version 3.12.0
locale:
[1] LC_CTYPE=C.UTF-8 LC_NUMERIC=C LC_TIME=C.UTF-8
[4] LC_COLLATE=C.UTF-8 LC_MONETARY=C.UTF-8 LC_MESSAGES=C.UTF-8
[7] LC_PAPER=C.UTF-8 LC_NAME=C LC_ADDRESS=C
[10] LC_TELEPHONE=C LC_MEASUREMENT=C.UTF-8 LC_IDENTIFICATION=C
time zone: UTC
tzcode source: system (glibc)
attached base packages:
[1] stats graphics grDevices utils datasets methods base
other attached packages:
[1] knitr_1.50 lubridate_1.9.4 forcats_1.0.0
[4] stringr_1.5.1 dplyr_1.1.4 purrr_1.0.4
[7] readr_2.1.5 tidyr_1.3.1 tibble_3.3.0
[10] tidyverse_2.0.0 broom_1.0.8 MASS_7.3-65
[13] ordinal_2023.12-4.1 robustbase_0.99-4-1 brms_2.22.0
[16] Rcpp_1.1.0 interactions_1.2.0 miceadds_3.17-44
[19] mice_3.18.0 lavaan_0.6-19 performance_0.14.0
[22] lme4_1.1-37 Matrix_1.7-3 caret_7.0-1
[25] lattice_0.22-7 ggplot2_3.5.2 car_3.1-3
[28] carData_3.0-5 rms_8.0-0 Hmisc_5.2-3
[31] petersenlab_1.1.6
loaded via a namespace (and not attached):
[1] splines_4.5.1 polspline_1.1.25 cellranger_1.1.0
[4] datawizard_1.1.0 hardhat_1.4.1 pROC_1.18.5
[7] rpart_4.1.24 lifecycle_1.0.4 Rdpack_2.6.4
[10] StanHeaders_2.32.10 processx_3.8.6 globals_0.18.0
[13] insight_1.3.1 backports_1.5.0 magrittr_2.0.3
[16] rmarkdown_2.29 yaml_2.3.10 pkgbuild_1.4.8
[19] gld_2.6.7 DBI_1.2.3 minqa_1.2.8
[22] RColorBrewer_1.1-3 multcomp_1.4-28 abind_1.4-8
[25] expm_1.0-0 quadprog_1.5-8 nnet_7.3-20
[28] TH.data_1.1-3 tensorA_0.36.2.1 sandwich_3.1-1
[31] jtools_2.3.0 ipred_0.9-15 lava_1.8.1
[34] inline_0.3.21 listenv_0.9.1 MatrixModels_0.5-4
[37] bridgesampling_1.1-2 parallelly_1.45.0 codetools_0.2-20
[40] tidyselect_1.2.1 shape_1.4.6.1 bayesplot_1.13.0
[43] farver_2.1.2 broom.mixed_0.2.9.6 effectsize_1.0.1
[46] matrixStats_1.5.0 stats4_4.5.1 base64enc_0.1-3
[49] jsonlite_2.0.0 e1071_1.7-16 mitml_0.4-5
[52] Formula_1.2-5 survival_3.8-3 iterators_1.0.14
[55] emmeans_1.11.1 foreach_1.5.2 tools_4.5.1
[58] DescTools_0.99.60 glue_1.8.0 mnormt_2.1.1
[61] prodlim_2025.04.28 gridExtra_2.3 pan_1.9
[64] mgcv_1.9-3 xfun_0.52 distributional_0.5.0
[67] loo_2.8.0 withr_3.0.2 numDeriv_2016.8-1.1
[70] fastmap_1.2.0 mitools_2.4 boot_1.3-31
[73] SparseM_1.84-2 callr_3.7.6 digest_0.6.37
[76] timechange_0.3.0 R6_2.6.1 estimability_1.5.1
[79] colorspace_2.1-1 mix_1.0-13 generics_0.1.4
[82] data.table_1.17.6 recipes_1.3.1 class_7.3-23
[85] httr_1.4.7 htmlwidgets_1.6.4 parameters_0.26.0
[88] ModelMetrics_1.2.2.2 pkgconfig_2.0.3 Exact_3.3
[91] gtable_0.3.6 timeDate_4041.110 furrr_0.3.1
[94] htmltools_0.5.8.1 scales_1.4.0 lmom_3.2
[97] posterior_1.6.1 gower_1.0.2 reformulas_0.4.1
[100] rstudioapi_0.17.1 tzdb_0.5.0 reshape2_1.4.4
[103] curl_6.4.0 coda_0.19-4.1 checkmate_2.3.2
[106] nlme_3.1-168 nloptr_2.2.1 proxy_0.4-27
[109] zoo_1.8-14 rootSolve_1.8.2.4 parallel_4.5.1
[112] foreign_0.8-90 pillar_1.11.0 grid_4.5.1
[115] vctrs_0.6.5 jomo_2.7-6 ucminf_1.2.2
[118] xtable_1.8-4 cluster_2.1.8.1 htmlTable_2.4.3
[121] evaluate_1.0.4 pbivnorm_0.6.0 mvtnorm_1.3-3
[124] cli_3.6.5 compiler_4.5.1 rlang_1.1.6
[127] rstantools_2.4.0 future.apply_1.20.0 labeling_0.4.3
[130] ps_1.9.1 fs_1.6.6 plyr_1.8.9
[133] rstan_2.32.7 stringi_1.8.7 psych_2.5.6
[136] pander_0.6.6 QuickJSR_1.8.0 viridisLite_0.4.2
[139] V8_6.0.4 glmnet_4.1-9 Brobdingnag_1.2-9
[142] bayestestR_0.16.1 quantreg_6.1 hms_1.1.3
[145] future_1.58.0 haven_2.5.5 rbibutils_2.3
[148] RcppParallel_5.1.10 DEoptimR_1.1-3-1 readxl_1.4.5