----- Forwarded message from
Soledad Esteban
Date: Sat, 24 Nov
2012 03:05:55 -0500
From: Soledad Esteban
Reply-To: Soledad Esteban
Subject: Re:
Do you need to know size for Procrustes superimposition?
To:
[email protected]
I hope this
helps!
Area de
Paleobiología
Institut Català de Paleontologia
Edifici ICP, Campus de la
UAB
08193 Cerdanyola del Vallès
Barcelona.
Spain
www.icp.cat
-----Mensaje original-----
De:
[email protected]
A: [email protected]
Fecha:
11/24/12 07:38
Asunto: Do you need to know size for Procrustes
superimposition?
----- Forwarded message from Patrick Kennedy -----
Date: Wed, 21
Nov 2012 21:19:26 -0500
From: Patrick Kennedy
Reply-To: Patrick
Kennedy
Subject: Do you need to know size for Procrustes
superimposition?
To: "[email protected]"
----- End forwarded message -----
----- End forwarded message
-----
Dear Patrick:
You do not need to know the real size
of the individuals to do a Procrustes superimposition, it just would compute the
centroid size from the relative picture size and scale all the individuals to
the same centroid size. With this you remove differences due to differences in
scaling when taking the pictures, but if there are some differences in shape
related to differences in size (allometry) they would be present in your sample
after superimpose them despite your specimens are fitted to the same size. The
only way of get ride of allometric scaling is knowing the real size of each
specimen (thus centroid size really represents the size of each specimen). In
biology allometry many times is quite an important factor, and usually it is
better to get the real size information whenever it is possible and check for
the importance of allometry on them. Indeed, depending on your study reviewers
may ask you to control for allometric factors, i.e.: are the shape differences
you get due to differences in size? And if yes, you may want to remove the
allometric effect working with the residuals of a regression between shape and
centroid size (or log centroid size). Thus, the real size is not important for
GPA, but it may be for your biological conclusions.
Therefore it is up
to you wether you take the risk and ignore the size factor which you may will
need later, but no need of it just for GPA.
Best regards
Sole
Soledad De Esteban Trivigno
Hi,
I'm completely new to morphometrics, so sorry if this question seems a bit
basic/obvious to those in the know!
I have a roughly 400 individuals, which are divided into nine different
groups, and I want to find out whether these different groups have different
mean shapes. I'm going to use MANOVA here. But to get to that point, I'll need,
of course, to do Procrustes superimposition. Each group was photographed at a
different time, so the scale differs each time. I have a reference to scale each
group to, but it would be a long and tedious process to go through each
individual setting the scale. Is this necessary? Could I just run the Procrustes
superimposition as it is with individuals photographed at different
distances?
In other words (and sorry if this seems a stupid question), do I need to
tell the software the actual/relative sizes of the specimens in order to do the
superimposition? I can't really see why you would need to (though I am a humble
undergraduate just beginning here, so I thought I would check).
Many thanks to anyone who can advise.
Patrick
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