[R] qqplot

[carol white]

Thanks for all your replies.

I just wanted to compare the distributions of two populations and see how different they are. I thought that QQplot would be a good idea. It's true that the mean and sd of the two distributions are not the same. I don't know if any other method or graphical presentation like five number, mean and sd as suggested, or boxplot would have been more suited to be used.

thanks for your advices,

--- On Mon, 11/2/09, John Fox <jfox at mcmaster.ca> wrote:

> From: John Fox <jfox at mcmaster.ca>
> Subject: RE: [R] qqplot
> To: "'Peter Flom'" <peterflomconsulting at mindspring.com>
> Cc: r-help at stat.math.ethz.ch, "'carol white'" <wht_crl at yahoo.com>, "'Yihui Xie'" <xieyihui at gmail.com>
> Date: Monday, November 2, 2009, 9:24 AM
> 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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> 
> 
>