[R] qqplot

[John Fox]

Dear Peter,

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.

Best,
 John


> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On
> Behalf Of Peter Flom
> Sent: November-02-09 11:57 AM
> To: carol white; Yihui Xie
> Cc: r-help at stat.math.ethz.ch
> Subject: Re: [R] qqplot
> 
> carol white <wht_crl at yahoo.com> wrote
> 
> >So the conclusion is that abline(0,1) should always be used and if it
> doesn't go through the qqplot, the two distributions are not similar?
> 
> I think it depends what you mean by "similar".  E.g., if you mean "are
both
> of these distributions (e.g.) normal?" then abline(0,1) is not always
useful.
> But if you mean "Do these have the same mean, sd, and distribution?" then
> abline(0,1) is the way to go.
> 
> 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
> 
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