Thank you for the explanation James. So that is why the ellipse is migrating towards the species 3 even though it does not have male species.
Regards, Helmi On Sunday, May 28, 2017 at 11:01:30 AM UTC+8, K. James Soda wrote: > > Dear Dr. Hadi, > > I would just like to add one additional comment to your question about the > need for normal distributions in PCA. Dr. Mitteroecker is (of course) > correct that PCA does not make any distributional assumptions, but you > mentioned in passing that confidence ellipses are being placed around the > points. Confidence ellipses usually do assume that the data is > multivariate normal. There are distribution-free methods for placing > confidence intervals, but my suspicion is that these intervals would not > usually have an elliptical appearance unless the data was in fact normally > distributed. > > So to reiterate: PCA, no distributional assumptions. Confidence > ellipses, usually assume multivariate normal data. > > Hope that comment is useful, > > James > > On Thu, May 25, 2017 at 4:58 AM, Helmi Hadi <[email protected] > <javascript:>> wrote: > >> Dear morphometricians, >> >> Does a sample need to be normally distributed when conducting PCA in >> geometric morphometrics? Sometimes due to research constraints there are no >> samples of the opposite sex. Someone was asking me this question, and I do >> not have the answer. When I look at the data distribution, there is quite >> an imbalance male/female population. However, the classifiers male/female >> and species are there and you can sort of tell which group belongs to >> where. My only fear is that the confidence ellipse for the males are being >> "gravitated" towards the females for one species as that species does not >> have any male specimens. Attached are the file which I have recreated the >> dataset based on memory. >> >> Is this kind of data acceptable or publishable? >> >> My own personal question is based on the GMM results given in MorphoJ. >> The PC1/PC2 axes does not intersect at the middle (which I have personally >> drawn the dotted line there). I don't mind this output, but does it matter >> to have the axes cut at the 0 value? The data data distribution does not >> change with the change of axes lines. I noticed some GMM papers have the >> axes at 0. >> >> Thanks all for the help, >> >> Helmi Hadi, >> School of Health Scienes, >> Universiti Sains Malaysia >> >> -- >> MORPHMET may be accessed via its webpage at http://www.morphometrics.org >> --- >> You received this message because you are subscribed to the Google Groups >> "MORPHMET" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to [email protected] <javascript:>. >> > > -- MORPHMET may be accessed via its webpage at http://www.morphometrics.org --- You received this message because you are subscribed to the Google Groups "MORPHMET" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected].
