Dear all,

I completely agree with the general consensus that the research question 
should inform the landmark sampling. As the first step in a morphometric 
study, landmark sampling is definitely worth thinking about deeply since, 
as discussed here and in many previous studies, it can generate spurious 
and unintended artifacts in alignment and downstream analyses. It's 
important to consider "quality over quantity" at both the individual 
landmark level and also at the level of the overall landmark configuration. 
For example, landmarks should be fairly evenly distributed on a structure 
of interest to avoid "Pinocchio effect" where isolated landmarks from the 
centroid end up having more impact on the alignment because the 
optimalization is based on *squared* distance from the centroid. 
Alternatively, one can use the Resistant Fit alignment to mitigate this 

Another note--my landmark sampling study shows that adding certain 
landmarks to a subsampled data set can sometimes decrease the overall fit 
to the full data set with complete set of landmarks. This result further 
supports the "quality over quantity" idea where choosing poor landmarks can 
lead to spurious characterization of shape variation, at least with respect 
to the full data set. Put in another way, adding more landmarks does not 
guarantee convergence to the full shape characterization (although it does 
typically converge from personal observation).

Happy landmarking,

On Tuesday, May 9, 2017 at 12:26:04 PM UTC+1, Lea Wolter wrote:
> Hello everyone, 
> I am new in the field of geometric morphometrics and have a question for 
> my bachelor thesis. 
> I am not sure how many landmarks I should use at most in regard to the 
> sample size. I have a sample of about 22 individuals per population or 
> maybe a bit less (using sternum and epigyne of spiders) with 5 populations. 
> I have read a paper in which they use 18 landmarks with an even lower 
> sample size (3 populations with 20 individuals, 1 with 10). But I have also 
> heard that I should use twice as much individuals per population as land 
> marks... 
> Maybe there is some mathematical formula for it to know if it would be 
> statistically significant? Could you recommend some paper? 
> Because of the symmetry of the epigyne I am now thinking of using just one 
> half of it for setting landmarks (so I get 5 instead of 9 landmarks). For 
> the sternum I thought about 7 or 9 landmarks, so at most I would also get 
> 18 landmarks like in the paper. 
> I would also like to use two type specimens in the analysis, but I have 
> just this one individual per population... would it be totally nonesens in 
> a statistical point of view? 
> Thanks very much for your help! 
> Best regards 
> Lea

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