In R, why do the default settings of
qqplot(linear model) use the standardized residuals on the y-axis? Why doesn't R use the "regular" residuals?
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When you use the standardized residuals, the expected value of the residuals is zero and the variance is (approximately) one. This has two benefits:
The theoretical residuals in a linear model are independent identically normally distributed. However the observed residuals are not independent and do not have equal variances. So standardizing the residuals divides by the estimated standard deviation associated with that residual making them more equal in their variances (using information from the hat matrix to compute this). This is a more meaningful residual to look at in the qqplot.
Also, are you really running qqplot on the fitted model? or is this the qqplot from runing plot on the model?