-------- Original Message --------
Subject: Re: Procrustes distance
Date: Mon, 3 Nov 2008 06:01:12 -0800 (PST)
From: Dennis E. Slice <[EMAIL PROTECTED]>
To: [email protected]
References: <[EMAIL PROTECTED]>
The concept of "big" depends, of course, on the within-sample
variability, hence the need for statistical tests.
That said, Procrustes distance is constrained to be <=pi/2 or about 1.57
radians (90 degrees) or sqrt(2) as a cord distance. A Procrustes
distance of 0.031 is about pi/100 (~1.8 degrees) whether in radians or
cord distance.
The largest zoologically meaningful Procrustes distance of which I am
aware was reported by Marcus et al. (2000) who compared the orders of
living mammals using cranial landmarks. The dolphin-to-muskrat
comparison produced a Procrustes distance of 0.731 radians (~42 degrees,
or ~pi/4). But note, the constraints of finding common landmarks on such
taxonomically disparate organisms likely reduces the amount of true
"shape" difference - whatever that means. This was recognized by Marcus
and colleagues.
-ds
Markus, L. F., E. Hingst-Zaher, H. Zaher. 2000. Application of landmark
morphometrics to skulls representing the orders of living mammals.
Hystrix 11(1): 27-47.
morphmet wrote:
-------- Original Message --------
Subject: Procrustes distance
Date: Mon, 3 Nov 2008 04:32:04 -0800 (PST)
From: Rebeca <[EMAIL PROTECTED]>
To: <[email protected]>
Dear Morphometricians,
I am studying shape variation in four populations of fish coming from
different geographical areas using a landmark- based GM approach.
I am doing permutation tests to check for mean shape differences between
the populations using the Procrustes distances, and a question about
these distances came up: Can we tell if a Proc. distance is fairly big
or small by seeing just the numbers? For example, if the distance
between two mean shapes is 0.031, is this a fairly big difference or not?
My question is more like when we describe the variation in the data, we
have a mean and a standard deviation, if the mean is 34.5 cm, and the sd
is 12 cm, I would say, by looking at the numbers that the sd is quite
‘big’.
Considering I am not an expert in morphometrics nor statistics, I hope
someone can help me to understand better this issue!
Thanks in advance,
Rebeca
Rebeca P. Rodriguez Mendoza
Email: [EMAIL PROTECTED]
Marine Research Institute CSIC
– Fisheries Dept.
C/ Eduardo Cabello, 6
Tel. 986 23 19 30 ext. 254
Fax: 986 29 27 62
36 208 Vigo, ESPAÑA
--
Dennis E. Slice
Associate Professor
Dept. of Scientific Computing
Florida State University
Dirac Science Library
Tallahassee, FL 32306-4120
-
Guest Professor
Department of Anthropology
University of Vienna
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