Dear Morphometricians I am new in geometric morphometrics. I have a question perhaps quite simple with regards to 2D GM. I am comparing the size/shape of human skulls from different sources using photographs taken in norma lateralis. One part was obtained through a standardized protocol while the other part was obtained from published pictures (with permissions) which were obtained using different protocols. I have traditional measurements for each skull for the published data set in order to compute the centroid size. However, taking into account some theoretical issues I feel that such comparison may be biased especially regarding size (i.e. CS) because both sets of images were obtained using different distances between the camera and the target. In order to minimize the size bias I plan to digitize landmarks and semilandmarks only along the skull contour given that its shape can be optimally characterized using a 2D approach. The location of landmarks in other structures poorly characterized from the 2D approach (i.e distinct to the contour) would increase the size bias. I will do a procrustes analysis using both data sets and through a multivariate regression of size on the procustes coordinates I will obtain residuals (i.e. shape variables). To my knowledge using this approach I will minimize the size bias mentioned. Obviously I will not use the centroid size in any subsequent analysis, only the shape variables. Does anyone tell me if this approach is right? Some suggestions? thanks in advance
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