[R] qqnorm(), is it "backwards"?
dmurdoch at pair.com
Fri Sep 22 02:49:42 CEST 2000
On Thu, 21 Sep 2000 17:10:27 -0500 (CDT), Pete Hurd
<phurd at uts.cc.utexas.edu> 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().
I think the problem is that there are several different conventions
for drawing these. Some people put the theoretical quantiles on the x
axis (like R does), some put them on the y axis. I think some people
plot probabilities rather than quantiles. Some books (e.g. if I
recall correctly, Montgomery and Peck's regression text) even label
things one way and draw them another.
As far as I can tell, R is doing things correctly according to the
convention it uses. That convention makes sense to me, too: like all
plots of residuals, the residuals are shown on the y axis, the "fixed
things" on the x axis.
For a sample like runif(1000), the plot curves up at the left and down
at the right, because the uniform distribution doesn't take such
extreme values as the normal distribution does.
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