Normal Q-Q plot in R is easily misinterpreted

Many R users don’t know this and they could be misinterpreting the R QQ plot.

Here is an example QQ plot from R, generated from an object of type aov named ‘fit’:

plot(fit, which=2)

Now there is a line on this image, and one might naturally assume that this line represents the perfect match with the standard normal, leading to one interpretation.

But let’s try plotting the 45 degree line representing a perfect match:

plot(fit, which=2)

abline(0,1)

Woah, quite a different interpretation.

The line drawn by default is not the 45 degree line, but rather the line “which passes through the first and third quartiles”.

 
See “?qqplot” for more info.

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