Razib Khan One-stop-shopping for all of my content

August 23, 2010

Just pushing buttons

Filed under: Analysis,Genetics,PCA,Tools — Razib Khan @ 11:05 pm

Mike the Mad Biologist, whose bailiwick is the domain of the small, asks in the comments:

I don’t mean to bring up a tangential point to the post, but why does the field of human genetics use PCA to visualize relationships? When I see plots like those shown here that have a ‘geometric pattern’ to them (the sharp right angles; another common pattern is a Y-shape), that tells me that there are lots of samples with zeros for many of the Y-variables (i.e., alleles that are unique to certain populations). Thus, the spatial arrangement of the points is largely an artifact of an inappropriate method: how does one calculate a correlation matrix when many of things one is correlating have values of zero?

If one really was keen on using PCA, one could calculate a pairwise distance matrix and then use that instead of the correlation matrix (Principal Coordinates Analysis).

Since I know some human geneticists do read this weblog, I thought it was worth throwing the question out there.

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