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

Duncan Murdoch 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.

Duncan Murdoch
r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html
Send "info", "help", or "[un]subscribe"
(in the "body", not the subject !)  To: r-help-request at stat.math.ethz.ch

More information about the R-help mailing list