-------- Original Message --------
Subject: Re: Calculating Consensus Shapes
Date: Fri, 14 Mar 2008 00:11:39 -0700 (PDT)
From: andrea cardini <[EMAIL PROTECTED]>
To: [email protected]
Dear Thomas,
there's little doubt, I dare say, that sample size matters when you do
the stats. Especially when studying small differences, as it often
happens with morphometrics, parametric stats is hard to do with small
samples. One can, somewhat, get around this using resampling stats but
statistical power tends to drop and SE tends to increase as N becomes
smaller. Also, results involving small samples often have low
reproducibility and to my experience it is not uncommon that mean shapes
from the smallest samples tend to behave like outliers. I find it very
informative to provide CI and SE whenever possible (for instance using
bootstraps).
Together with Sarah Elton, we explored a little bit this kind of issues
in a study published last year:
CARDINI A., Elton S., 2007 - Sample size and sampling error in geometric
morphometric studies of size and shape. Zoomorphology, 126: 121-134.
Also, there are bits on the same stuff in some recent papers in JHE and
Biol J Linn Soc.
If you're interested, I can send you reprints.
Cheers
Andrea
At 22:27 13/03/2008 -0400, you wrote:
-------- Original Message --------
Subject: Calculating Consensus Shapes
Date: Thu, 13 Mar 2008 14:37:58 -0700 (PDT)
From: Greiner Thomas <[EMAIL PROTECTED]>
To: <[email protected]>
When comparing shapes among different groups we usually reference each
individual, or the consensus shape of each group, to the grand consensus
shape of the data pool. But, what do we do if our groups are of grossly
unequal sample sizes? If this were a univariate problem I would be
tempted to use weighted means for each group. But, how do we do this
with multivariate shape data?
Is it important to account for sample size differences when calculating
PCA scores? What about when creating the spline deformation grids?
Do the common programs that provide consensus shapes already account for
potential sample size differences?
Is this not as big a problem as I am thinking it might be (I've got
groups with sample sizes ranging from 300 to 20)?
*/Thomas M. Greiner, Ph.D./*
Anatomist and Physical Anthropologist
Dept. of Health Professions
University of Wisconsin - La Crosse
1725 State Street
La Crosse, WI 54601 USA
Phone: (608) 785-8476
Fax: (608) 785-8460
--
Replies will be sent to the list.
For more information visit http://www.morphometrics.org
Dr. Andrea Cardini
Lecturer in Animal Biology
Museo di Paleobiologia e dell'Orto Botanico, Universitá di Modena e Reggio
Emilia
via Università 4, 41100, Modena, Italy
tel: 0039 059 2056532; fax: 0039 059 2056535
Honorary Fellow
Functional Morphology and Evolution Unit, Hull York Medical School
University of Hull, Cottingham Road, Hull, HU6 7RX, UK
University of York, Heslington, York YO10 5DD, UK
E-mail address: [EMAIL PROTECTED], [EMAIL PROTECTED],
[EMAIL PROTECTED]
http://hyms.fme.googlepages.com/drandreacardini
http://ads.ahds.ac.uk/catalogue/archive/cerco_lt_2007/overview.cfm#metadata
--
Replies will be sent to the list.
For more information visit http://www.morphometrics.org
