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

Pete Hurd phurd at uts.cc.utexas.edu
Fri Sep 22 00:10:27 CEST 2000

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


Peter L. Hurd, Ph.D.
phurd at uts.cc.utexas.edu
http://www.zo.utexas.edu/research/phurd                 fax 512.471-3878
Section of Integrative Biology, University of Texas, Austin TX 78712 USA

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