qqplot
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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