Thanks very much for all the helpfull replies you have provided to my request
Thomas morphmet wrote: >Thomas, > >The model you described is MANCOVA, with groups, size, and group*size >interaction as the main model effects. When the interaction term of a >MANCOVA is significant, asking the question of group difference via the >main effect is no longer useful, because the allometric relationships >are not concordant among the groups themselves. Further, you cannot >simply adjust the shape data for each group separately, as you are then >comparing residual shape based on different allometric relationships for >each group. Instead, it is preferable to alter the way you think about >your question so that it more closely aligns with the biological >patterns that you observe in your data. > >When the interaction term is significant, this effectively shifts the >group difference question away from 'pure' shape differences, and >instead to determining HOW the allometric trajectories themselves >differ. It turns out that there are several ways that the interaction >term could be found significant; all of which are biologically >interesting. First, the slopes could differ among groups. This is >equivalent to having allometric trajectories pointing in different >directions in shape space. However, it may be the case that the amount >of shape change associated with size may differ among groups. This is >equivalent to the magnitude of shape change being different among >groups, which will also exhibit a significant interaction term. Of >course, both of these may be occurring in your data set. Finally, the >shape of the allometric trajectories could also differ, but you would >not identify that unless you are using non-linear allometric procedures. > >To determine which of these aspects of allometric shape change differ in >your data set requires tests of both magnitude and direction. The >analytical procedure is described in: > >Collyer & Adams. 2007. Analysis of two-state multivariate phenotypic >change in ecological studies. /Ecology./ 88:683-692. > >The paper describes the scenario for 2 categorical factors, but the >approach for a continuous covariate is identical. For extensions and >related topics see also: > >Adams, and Collyer. 2007. Analysis of character divergence along >environmental gradients and other covariates. Evolution.61:510-515. >Adams, and Cerney. 2007. Quantifying biomechanical motion using >Procrustes motion analysis. Journal of Biomechanics. 40:437-444. > >Hope this helps. > >Dean > >Dean C. Adams >Associate Professor >Department of Ecology, Evolution, and Organismal Biology, and > Department of Statistics >253 Bessey Hall >Iowa State University >Ames, IA 50011 >tel: (515) 294-3834 >fax: (515) 294-1337 >web: http://www.public.iastate.edu/~dcadams > >At 03:01 PM 8/23/2007, you wrote: > > > >>Dear morphometricians, >> >>I work on the Origin of Orcadian voles using a GPA analysis on first >>lower molar (M1) Landmarks. >>I use TPS series and NTSYS-pc to perform the analyses. >>Voles' M1 display huge differences in their centroid size which has, >>according to the multivariate regression, highly significant effect over >>shape variance. >>I would like to remove this allometric component. Is it possible to >>perfom a standardization with TPSregr? >> >>Thank you very much in advance for any kind of help >> >>Thomas >> >>-- >>Dr Thomas Cucchi >>Postdoctoral Research Associate >>Durham University >>Department of Archaeology >>South road >>Durham >>DH1 3LE >> >>Tel: >>00 44 (0) 191 33 41162 >> >>e-mail: >>[EMAIL PROTECTED] >> >>-- >>Replies will be sent to the list. >>For more information visit http://www.morphometrics.org >><http://www.morphometrics.org/> >> >> > > > -- Dr Thomas Cucchi Postdoctoral Research Associate Durham University Department of Archaeology South road Durham DH1 3LE Tel: 00 44 (0) 191 33 41162 e-mail: [EMAIL PROTECTED] -- Replies will be sent to the list. For more information visit http://www.morphometrics.org
