-------- Original Message --------
Subject: Re: Analyzing ontogenetic trajectory angles
Date: Tue, 19 Jul 2011 14:40:12 -0400
From: Cabo-Perez, Luis <[email protected]>
To: [email protected] <[email protected]>
David, can you suggest any references for the original method that you
are describing? I am not familiar with it and, from your description, I
find the problem very interesting. At first sight, I am with you on all
your points. Plus one: what does the angle provide, if you already have
the slopes? Wouldn't that be like calculating a ratio from the slopes,
rather than using them directly?
Please, if you do not mind, keep me updated of the answer that you get,
if they are not posted in morphmet.
Thanks,
Luis
Luis Cabo
Department of Applied Forensic Sciences
Mercyhurst Archaeological Institute,
Mercyhurst College, Erie, PA
E-mail: [email protected]
-------- Original Message --------
Subject: Analyzing ontogenetic trajectory angles
Date: Mon, 18 Jul 2011 14:46:19 -0400
From: David Katz <[email protected]>
To: [email protected]
Hello,
I have read several morphometrics papers which test for significant
differences in ontogenetic trajectory between two groups (species,
subspecies, etc) by calculating the "angle" between their growth
trajectories. However, parts of (or even lots of) the method remain
unclear to me.
First, it seems that calculation of the angle requires calculation of
two simple regression lines, one for each group, with the angle being
the arc subtended by the two lines. One axis for these regression
plots/calculations is the distribution of specimens along a PC which is
significantly correlated with size or age (usually the first PC). But
it is not clear to me what the second axis is. Log centroid size?
Second, the significance of the angle is tested by randomly permuting
specimens between the two groups 1000 or more times, then calculating a
new angle for each permutation. Significance is then tested against the
distribution of permutation-generated angles. But what is the
permutation procedure? Do we only permute a single specimen each time?
Third, if I'm understanding the method correctly, then absent an
additional scaling step, the two lines from which the angle is
calculated are not likely to be of the same length. How is this
accounted for, if at all?
Any help, or a reference to a good, explicit journal or book
explanation, would be very much appreciated.
Thanks.
David Katz
Doctoral Candidate
Department of Anthropology--Evolutionary Wing
University of California, Davis
Young Hall 204
/--Trying to focus on one distraction at a time/--
