Hi Philipp, I am not worried about the number of variables (although I am not sure one needs thousands of highly correlated points on a relatively simple structure and seem to remember that Gunz and you suggest to start with many and then reduce as appropriate).
Regardless of whether point homology makes sense, I am worried that many users believe that semilandmarks (maybe after sliding according to purely mathematical principles) are the same as "traditional landmarks" with a clear one-to-one correspondence. Even saying that what's "homologous" is the curve or surface is tricky, because at the end of the day that curve/surface is discretized using points, shape distances are based on those points and there are many ways of placing points with no clear "homology" (figure 7 of Oxnard & O'Higgins, 2009); indeed, in a ontogenetic study of the cranial vault, for instance, where sutures may become invisible in adults and therefore cannot be used as a "boundary", semilandmarks close to the sutures may end up on different bones in different stages/individuals. Semilandmarks are a fantastic tool, which I am happy to use when needed, but they have their own limitations, which one should be aware of. Cheers Andrea On 03/06/2017, mitte...@univie.ac.at <mitte...@univie.ac.at> wrote: > I think a few topics get mixed up here. > > Of course, a sample can be too small to be representative (as in Andrea's > example), and one should think carefully about the measures to take. It is > also clear that an increase in sample size reduces standard errors of > statistical estimates, including that of a covariance matrix and its > eigenvalues. But, as mentioned by Dean, the standard errors of the > eigenvalues are of secondary interest in PCA. > > If one has a clear expectation about the signal in the data - and if one > does not aim at new discoveries - a few specific measurements may suffice, > perhaps even a few distance measurements. But effective exploratory > analyses have always been a major strength of geometric morphometrics, > enabled by the powerful visualization methods together with the large > number of measured variables. > > Andrea, I am actually curious what worries you if one "collects between > 2700 and 10 400 homologous landmarks from each rib" (whatever the term > "homologous" is supposed to mean here)? > > Compared to many other disciplines in contemporary biology and biomedicine, > > a few thousand variables are not particularly many. Consider, for instance, > > 2D and 3D image analysis, FEA, and all the "omics", with millions and > billions of variables. In my opinion, the challenge with these "big data" > is not statistical power in testing a signal, but finding the signal - the > low-dimensional subspace of interest - in the fist place. But this applies > to 50 or 100 variables as well, not only to thousands or millions. If no > prior expectation about this signal existed (which the mere presence of so > many variables usually implies), no hypothesis test should be performed at > all. The ignorance of this rule is one of the main reasons why so many GWAS > > and voxel-based morphometry studies fail to be replicable. > > Best wishes, > > Philipp > > -- > 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 morphmet+unsubscr...@morphometrics.org. > -- Dr. Andrea Cardini Researcher, Dipartimento di Scienze Chimiche e Geologiche, Università di Modena e Reggio Emilia, Via Campi, 103 - 41125 Modena - Italy tel. 0039 059 2058472 Adjunct Associate Professor, School of Anatomy, Physiology and Human Biology, The University of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia E-mail address: alcard...@gmail.com, andrea.card...@unimore.it WEBPAGE: https://sites.google.com/site/alcardini/home/main FREE Yellow BOOK on Geometric Morphometrics: http://www.italian-journal-of-mammalogy.it/public/journals/3/issue_241_complete_100.pdf ESTIMATE YOUR GLOBAL FOOTPRINT: http://www.footprintnetwork.org/en/index.php/GFN/page/calculators/ -- 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 morphmet+unsubscr...@morphometrics.org.