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