----- Forwarded message from a.vanhete...@roehampton.ac.uk -----
Date: Thu, 27 Sep 2012 04:03:46 -0400
From: a.vanhete...@roehampton.ac.uk
Reply-To: a.vanhete...@roehampton.ac.uk
Subject: RE: PCA of landmark data on wings
To: morphmet@morphometrics.org
Dear Jason
The mirror effect is just an artifact and does not mean anything. It is
probably due to rounding of the data done by the programs you use. I often have
mirrored axes when I perform the exact same analysis in different computer
programs. It is nothing to worry about and it also does not effect the
interpretation of your data. Specimens that are on opposite sides of you axis
are very different from each other (with respect to the features representing
that axis) and specimens that have very similar scores on that axis are very
similar (again with respect to the features representing that axis). If the
specimens are reversed on your PC axis, the features determining that PC axis
(say, round vs elongated wings) will also be reversed, so your conclusions
should be the same.
I hope that helps.
Best wishes,
Anneke van Heteren
________________________________________
From: morphmet_modera...@morphometrics.org
[morphmet_modera...@morphometrics.org]
Sent: 26 September 2012 21:48
To: morphmet@morphometrics.org
Subject: PCA of landmark data on wings
----- Forwarded message from Jason Mottern -----
Date: Wed, 26 Sep 2012 12:50:47 -0400
From: Jason Mottern
Reply-To: Jason Mottern
Subject: PCA of landmark data on wings
To: morphmet@morphometrics.org
Hello,
I am a new list member, as well as a novice with respect to geometric
morphometric analysis. I am doing a morphometric analysis of landmark data on
wasp wings, and I'm doing both PCA and CVA. I am analyzing males and females
separately, and there are three species. When I run all four analyses (CVA and
PCA for each sex), the directions along the principal component axes are
reversed for the female PCA analysis only. In other words, the signs are all
reversed on the PC scores relative to the other three analyses, so its graph is
"mirrored" compared to the other three. I am most intrigued as to why this
reversal occurs between the female PCA and female CVA. These two analysis are
based on the exact same set of partial warp scores, though, of course,
subsequent calculations are different. I don't understand how the directions of
the vectors are determined, and why they might differ between a PCA and CVA
analysis of the same data. The programs I'm using for the analyses are PCAG!
en and CVAGen (Sheets, 2002). I apologize if I'm not articulating the
phenomenon very well, but, like I said, I'm very new to this stuff. Any
thoughts would be greatly appreciated.
Cheers,
Jason Mottern
jmott...@ucr.edu<http://mailto:jmott...@ucr.edu>
----- End forwarded message -----
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