Dear MorphMetters, Some of you may have been in the auditorium in the Department of Botany, University of Vienna, back in March when Philipp Mitteroecker and I were the two scheduled discussants for the conference "GMAustria19" on applications of geometric morphometrics. Several of the papers delivered there used between-group principal components analysis (bgPCA), and after each of those papers I mentioned in the course of my commentary that bgPCA was fatally flawed in applications to most GMM data sets and should NEVER be used here. In my keynote address, which closed the meeting, I had one cryptic slide about this assertion, with an example that flashed on the screen but was immediately replaced by the next slide. The typical response to both my own talk and my criticism of the talks of others, as far as bgPCA was concerned, was along the lines of "Hunh?" or sometimes "What are you blathering about this time? Isn't bgPCA in the standard toolkit?" I answered that the Bookstein paper they should read was just then being written, as one of a pair jointly arising from conversations with Andrea Cardini, Jim Rohlf, and Paul O'Higgins following an original hunch of Cardini's, and that my argument would be pretty convincing once it was actually written down. The claim isn't that people are using bgPCA incorrectly. They're using it according to the published formulas, yes, but the method itself yields biological nonsense much too often. That was March. In April, two different articles in Nature (one by Detroit et al., one by Chen et al.) buttressed claims about sister species of Homo sapiens using the bgPCA method, and so suddenly it became clear that we authors had to do something quickly lest this become an epidemic of bad biometrics. So we accelerated our writing. My paper was the first to be finished, probably because it is a single-authored item by an emeritus with no other obligations, and it seemed like a good idea to upload the final draft to https://www.biorxiv.org even before submitting the paper, so that any letter to the editors of Nature could include a link to the argument as to exactly WHY bgPCA is nearly always unsound and its inferences invalid for applications in contemporary GMM. That is the draft that has just appeared as
https://www.biorxiv.org/content/10.1101/627448v1 For those of you who were at the March meeting, this is the argument (complete with formulas) defending my stern condemnation there. I won't try to summarize it in this morphmet note -- if you're interested, just read the abstract on page 1 of the link. For those of you who have already published bgPCA analyses, you know who you are -- my paper argues strongly that you need to go back and revisit the inferences of those papers in a mood of much more intense multivariate skepticism. For the rest of you, please consider this draft manuscript to be a wake-up call. A technique that has appeared in dozens of papers and that was, alas, specifically praised by Mitteroecker and Bookstein personally (back in 2011) could nevertheless, when examined closely (for the first time!), turn out to be algebraic garbage when applied to data sets where there are far more shape coordinates than specimens. But isn't that the usual situation in GMM these days? As always, I welcome all responses, both positive and negative. The biorxiv posting is permanent, but there is plenty of time for me to make changes before the paper is published (at present it has not yet even been submitted anywhere), so feel free to try to find the flaws in my argument. But I hope you will want to try some of these simulations on your own before you argue against mine. You will also want to study the companion piece by Cardini, O'Higgins, and Rohlf that should likewise be available for download before too long. Fred Bookstein -- 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.