# Regression Diagnostics

An excellent review of regression diagnostics is provided in John Fox's aptly named Overview of Regression Diagnostics. Dr. Fox's **car** package provides advanced utilities for regression modeling.

`# Assume that we are fitting a multiple linear regression`

#
on the MTCARS data

library(car)

fit <- lm(mpg~disp+hp+wt+drat, data=mtcars)

This example is for **exposition only**. We will ignore the fact that this may not be a great way of modeling the this particular set of data!

## Outliers

`# Assessing Outliers`

outlierTest(fit) # Bonferonni p-value for most extreme obs

qqPlot(fit, main="QQ Plot") #qq plot for studentized resid

leveragePlots(fit) # leverage plots

## Influential Observations

`# Influential Observations`

# added variable plots

av.Plots(fit)

# Cook's D plot

# identify D values > 4/(n-k-1)

cutoff <- 4/((nrow(mtcars)-length(fit$coefficients)-2))

plot(fit, which=4, cook.levels=cutoff)

# Influence Plot

influencePlot(fit, id.method="identify", main="Influence Plot", sub="Circle size is proportial to Cook's Distance" )

## Non-normality

`# Normality of Residuals`

# qq plot for studentized resid

qqPlot(fit, main="QQ Plot")

# distribution of studentized residuals

library(MASS)

sresid <- studres(fit)

hist(sresid, freq=FALSE,

main="Distribution of Studentized Residuals")

xfit<-seq(min(sresid),max(sresid),length=40)

yfit<-dnorm(xfit)

lines(xfit, yfit)

## Non-constant Error Variance

`# Evaluate homoscedasticity`

# non-constant error variance test

ncvTest(fit)

# plot
studentized residuals vs. fitted values

spreadLevelPlot(fit)

## Multi-collinearity

`# Evaluate Collinearity`

vif(fit) # variance inflation factors

sqrt(vif(fit)) > 2 # problem?

## Nonlinearity

`# Evaluate Nonlinearity`

# component + residual plot

crPlots(fit)

# Ceres plots

ceresPlots(fit)

## Non-independence of Errors

`# Test for Autocorrelated Errors`

durbinWatsonTest(fit)

## Additional Diagnostic Help

The **gvlma( )** function in the **gvlma** package, performs a global validation of linear model assumptions as well separate evaluations of skewness, kurtosis, and heteroscedasticity.

`# Global test of model assumptions`

library(gvlma)

gvmodel <- gvlma(fit)

summary(gvmodel)

## Going Further

If you would like to delve deeper into regression diagnostics, two books written by John Fox can help: Applied regression analysis and generalized linear models (2nd ed) and An R and S-Plus companion to applied regression.