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 <helm...@gmail.com 
> <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
>>
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