[R-sig-phylo] How to detect phylogenetic signal (lambda) in one unscaled trait?
Joe Felsenstein
joe at gs.washington.edu
Tue Mar 22 23:56:24 CET 2011
Alberto Gallano wrote:
> I think I was not clear with what I said about the log transformation, and I
> see now what you mean about using log-log when using regression. Though It
> does seem to me that logging two variables in a ratio context:
>
> log(Y) / log(X)
>
> or
>
> log(Y / X)
>
> would influence phylogenetic signal levels greatly, since, while the order
> of trait means is maintained, the magnitude of differences between trait
> means is altered (means with higher values are dragged toward the centre of
> the distribution). I assume, then, that this is nothing to worry about.
The above two are *not* equivalent. If a trait is doing a Brownian
Motion on the log scale, the latter, log(Y/X) is OK, as it is equivalent
to to log(Y) - log(X), a straightforward linear combination.
But log(Y)/log(X) could get you into real trouble, particularly
if X got near 1, in which case the ratio could blow up.
Joe
----
Joe Felsenstein joe at gs.washington.edu
Department of Genome Sciences and Department of Biology,
University of Washington, Box 355065, Seattle, WA 98195-5065 USA
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