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).
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
> 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
MORPHMET may be accessed via its webpage at http://www.morphometrics.org
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