[R] qqnorm(), is it "backwards"?
Bill.Venables at cmis.csiro.au
Fri Sep 22 02:01:16 CEST 2000
At 17:10 21/09/00 -0500, Pete Hurd wrote:
>Hello R friends,
>I'm wondering why I get funny qqnorm() results. It seems that they should
>all be reflected in the normal qqline().
>For instance: if I qqnorm() bimodal or uniform data I get a sigmoidal in
>which the qqnorm() points lie above the qqline() at -ve theoretical
>quantiles, and the qqnorm() points lie below the qqline() at +ve
>theoretical quantiles. Yet I expect such platykurtic distributions to go
>the other way (eg pg 117 in _Biometry_ Sokal & Rohlf, 3rd ed).
>The same thing with skewed data, I expect right skewed data to show a
>negatively accelerating shape, but qqnorm() curves upwards.
>Am I missing something, or is qqnorm() consistently heading in the wrong
Yes, you are missing something.
Sokal and Rohlf do their normal scores plots the *wrong* way round and put
the rankits (or normal scores) on the y-axis and the observations on the
x-axis. To its credit R and S-PLUS get it *right* by default and put the
independent variable (normal scores) on the x-axis and the dependent
variable (observations, residuals, ...) on the y-axis.
Sokal and Rohlf are not alone in their *error*, of course. Minitab also
*messes* it up like this, for example. I have no idea where this absolute
*heresy* comes from, but someone really ought to do some serious
extermination of this pernicious silliness before it goes too far...
Bill Venables, Statistician Tel. +61 7 3826 7251
CSIRO Marine Laboratories, Fax. +61 7 3826 7304
Cleveland, Qld, 4163 Email: Bill.Venables at cmis.csiro.au
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