Variance Inflation Factor (VIF) is a common simple stat used to quantify multicollinearity in least squares regressions. It is calculated for each covariate in a regression, with higher values meaning that the covariate is more colinear with the other covariates. It technically measures “how much the variance (the square of the estimate’s standard deviation) of an estimated regression coefficient is increased because of collinearity.” The equation is:

where R2i is from the regression of the covariate i on all the other covariates. The problem is where to draw the cutoff? Is a VIF > 2.5 too high? >5? or how about VIF>10, all have been used as cutoffs. Here is a figure of R2 vs VIF. As you can see, a cuttoff of 2.5 is an R2 of 0.60 and 10 is 0.90! While statistically, you could perhaps get away with these high inflations, what does it mean for your particular question? If you are dealing with a relationship among covariates that is as strong as 0.90, can you really be sure that the model and your interpretations are valid?

`#VIF function`

r<-function(x){1-(1/x)} #r is R2 and x is VIF

x<-seq(1,15,.1) #seq of VIFs

y<-sapply(x,r) #seq of R2

#plot

par(las=1)

plot(x,y,type="l",xlab="VIF",ylab="R2 of regression of focal covariate on all other covariates")

# common VIF cutoffs = 2.5, 5, 10

ly<-c(y[x==2.5],y[x==5],y[x==10])

lx<-c(2.5,5,10)

segments(lx,0,lx,ly,col="red")

segments(lx,ly,0,ly,col="red")