qqplot

John Fox <jfox at mcmaster.ca> wrote

I assumed that Carol wanted to compare the shapes of the distributions and
to adjust for differences in centre and spread. To put a line through the
quartiles or to base a line on the medians and IQRs is more robust than
using the means and sds.

Hi John

Indeed it is.

It all depends on what she wants to use the QQplot for, and what she wants to adjust for.

But I would guess (and I haven't done any simuls) that the method with mean and sd will usually be pretty
close to the one with medians and IQRs.

Maybe I will look at it a bit.

Peter

Peter L. Flom, PhD
Statistical Consultant
Website: www DOT peterflomconsulting DOT com
Writing; http://www.associatedcontent.com/user/582880/peter_flom.html
Twitter:   @peterflom