procrustes variances

[morphmet]

-------- Original Message --------
Subject:        procrustes variances
Date:   Thu, 27 Aug 2009 07:41:39 -0700 (PDT)
From:   Louis Boell <lbo...@hotmail.de>
To:     <morphmet@morphometrics.org>



Dear colleagues,

I have a question about procrustes variance. I want to compare the shape
variances of different samples. I have three groups of 77, 96 and 17
specimens, respectively. I calculated the procrustes variance of each
group in two ways: 1) after pooling the raw data of all three groups
into a common total dataset and fitting them together; b) after
calculating the procrustes fit for each dataset/group separately.

The results for the two large samples are quite consistent between both
procedures; however, the estimate of the procrustes variance for the 17
specimen sample is much larger when fitted together with the other two
samples than when fitted separately.

I assume that this is because the procrustes fit is a "democratic"
procedure, which is much more influenced by large samples than by small
samples when they are fitted together. This could potentially result in
a "spreading" of the specimens from the specimens from the small sample
in the space of the procrustes coordinates, if their covariance pattern
is different from the mean covariance pattern of the total dataset which
will be largely determined by the large samples.

Altogether, my question amounts to whether it is more approriate to
compare procrustes variances from separate procrustes fits or from a
procrustes fit of the pooled total dataset.

Thanks in advance for help

Louis Boell



Louis Boell
MPI für Evolutionsbiologie
August-Thienemannstr.2
24306 Plön
lbo...@evolbio.mpg.de
lbo...@hotmail.de



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