[R] qqnorm(), is it "backwards"?

Bill Venables 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.
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
AUSTRALIA                        http://www.cmis.csiro.au/bill.venables/

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