source("http://www.uvm.edu/~rsingle/stat221/data/scripts-221.R")

 dat <- otherdata("resid1.dat")
 
#Compute VIF in 2 different ways
 y.x1x2 <- lm(Y~X1+X2, data=dat)
 summary(y.x1x2)$coef
 vif(y.x1x2)
 1/( 1 - cor(dat$X1,dat$X2)^2 )
 
#Create a highly collinear variable
 dat$X3 <- dat$X1
 dat$X3[1]
 dat$X3[1] <- 32

#Compare VIF values for the 2 models
 y.x1x2x3 <- lm(Y~X1+X2+X3, data=dat)
 vif(y.x1x2x3)
 summary(y.x1x2)$coef
 summary(y.x1x2x3)$coef
 2.2725308/0.2721941