-------- Original Message -------- Subject: Re: Comparing Segments of Developmental Trajectories Date: Tue, 13 Mar 2012 05:12:10 -0400 From: andrea cardini <alcard...@gmail.com> To: morphmet@morphometrics.org Hi Michelle, Sarah Elton and I, years ago, explored a little bit how variable angles were when sample size got smaller. We did not do the tests of significance but found quite a big variation. The reference is below and it is available also in my webpage: Cardini A., Elton S., 2007 - Sample size and sampling error in geometric morphometric studies of size and shape. Zoomorphology, 126: 121-134. My guess is that statistical power will be very low. Within a developmental stage, trajectories are not unlikely to be almost circular (with variation 'squeezed' just a little bit in a given direction): to get accurate angles, one will then need really big samples. Cobb and O'Higgins, if I remember well, also explored the issue with a similar approach but they were comparing ontogenetic trajectories (all stages) across species. Those trajectories will be much more 'stretched' and smaller samples might still be reasonably accurate. I can find the ref. but I am sure you already know the paper. Good luck. Cheers Andrea At 23:29 12/03/2012 +0100, you wrote:
-------- Original Message -------- Subject: Comparing Segments of Developmental Trajectories Date: Mon, 12 Mar 2012 18:15:09 -0400 From: Michelle Singleton <msingle...@midwestern.edu> To: morphmet@morphometrics.org Dear Colleagues, As part of a study of ontogenetic shape change in a group of related species, I wish to compare patterns of shape change between successive developmental stages. My intention was to compare angular differences between species vectors obtained from multivariate regression of Procrustes residuals on my developmental variable. When I apply this approach to the full developmental series (juvenile to adult) I get interspecies angles comparable to those obtained by myself and others in prior studies, but when I look at individual segments (e.g., Stage 1 to Stage 2) the resulting angles are very large, apparently because the amount of variation between stages is too small to allow accurate vector estimates, although the smaller sample sizes probably contribute as well. The large angles do, nevertheless, return the same qualitative result (in terms of relative vector similarity) as the angles for the full ontogenetic series. My questions are: 1) have I correctly identified the source of the discrepancy in angle magnitudes? 2) can permutation significance tests based on these angles be meaningful; or, 3) is this the wrong approach and is there perhaps a more appropriate method for this comparison? Many thanks for your thoughts on this problem. Best regards, Michelle -- Michelle Singleton, Ph.D. Professor of Anatomy Midwestern University 555 31st Street Downers Grove, IL 60515 Phone: 630.515.6137 <tel:630.515.6137> Fax: 630.515.7199 <tel:630.515.7199> e-mail: msingle...@midwestern.edu <mailto:msingle...@midwestern.edu>
Dr. Andrea Cardini Researcher in Animal Biology Dipartimento di Biologia, Universitá di Modena e Reggio Emilia, via Campi 213, 41100, Modena, Italy tel: 0039 059 2055017 ; fax: 0039 059 2055548 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 Adjunct Associate Professor Centre for Forensic Science , The University of Western Australia 35 Stirling Highway, Crawley WA 6009, Australia E-mail address: alcard...@gmail.com, andrea.card...@unimore.it, andrea.card...@hyms.ac.uk, andrea.card...@uwa.edu.au Webpage: http://sites.google.com/site/hymsfme/drandreacardini Datasets: http://ads.ahds.ac.uk/catalogue/archive/cerco_lt_2007/overview.cfm#metadata Editorial board for: Zoomorphology: http://www.springer.com/life+sciences/animal+sciences/journal/435 Journal of Zoological Systematics and Evolutionary Research: http://www.wiley.com/bw/journal.asp?ref=0947-5745&site=1 Hystrix, the Italian Journal of Mammalogy: http://www.italian-journal-of-mammalogy.it/
