# Why is R plotting standardized residuals against theoretical quantiles in a Q-Q plot?

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:

1. If you rescale one of your variables (say change kilometres to miles), the residual plots remain unchanged.
2. In the qqplot, the residuals should lie on the line y = x
3. You expect 95% of your residuals to lie between -1.96 and 1.96. This makes it easier to spot outliers.
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Could you please give me a reference, that 95% should lie between -1.96;1.96 . In other words, why should standardized residuals be $d_i\sim N(0,1)$? –  MarkDollar Jul 7 '11 at 15:42
@Mark: Just google "standardised residuals" and look at the first few hits. –  csgillespie Jul 8 '11 at 9:29