Date:         Wed, 10 Oct 2007 17:32:07 -0700
Reply-To:     sounpra@YAHOO.COM
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         Song <sounpra@YAHOO.COM>
Organization: http://groups.google.com
Subject:      Re: PCA using SAS and R
Comments: To: sas-l@uga.edu
In-Reply-To:  <18342042.1192051433551.JavaMail.root@mswamui-bichon.atl.sa.earthlink.net>
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On Oct 10, 4:23 pm, peterflomconsult...@mindspring.com (Peter Flom)
wrote:
> Song <soun...@YAHOO.COM> wrote
>
>
>
> >Can anyone please explain to me how running a simple PCA in SAS and R
> >can produce conflicting results?  You will notice that signs of the
> >component score are different only in the 4th component.  What is
> >troubling to me is that this is not consistent.  On a different
> >dataset, the signs become different starting with the 2nd component.
>
> The signs of the principal components are arbitrary.  It's like saying "John is taller than Mary" or "Mary is shorter than John".
>
> If the magnitudes were noticeably different, that would be troubling, but changes in the signs are no big deal
>
> Peter

Hi Peter --

I understand what you are saying...however this does matter if I'm
trying to construct biplots using say the first two PCs.  This
wouldn't be a problem if I knew a priori that the sign will differ
only after the second component...but it changes for different
dataset. So the biplots that I get from SAS and R may or may not be
the same.

Song