Rob Lanfear wrote: > In particular, Liam's using squared contrasts in y, so that's > asking whether the absolute size of changes in y depends on > x at the ancestral node. I might have missed something here, > but that sounds very similar in principle to Freckleton's > test of whether the variance of trait differences is > unrelated to their absolute values [1], except that the > latter looks for correlations between the absolute value of > differences in x versus x at the ancestral node. It might be > useful to consult that paper [1] to get some more ideas for > how to interpret those kinds of results.
If there is proportionality between standard deviation of change and the value of the character, that is essentially saying that the log(x) changes at rate independent of its value. Similarly, if the variance of change is proportional to the value of the character, that is essentially the same as saying that the square root of the character changes at a rate independent of its value. See this Wikipedia page: http://en.wikipedia.org/wiki/Variance-stabilizing_transformation Perhaps the whole test can be done by considering different transforms of the data (there are parameterized families of them) and use the likelihood to test values of the transform parameters (with appropriate correction of the likelihood by having a Jacobian term). Joe ---- Joe Felsenstein j...@gs.washington.edu Department of Genome Sciences and Department of Biology, University of Washington, Box 355065, Seattle, WA 98195-5065 USA _______________________________________________ R-sig-phylo mailing list - R-sig-phylo@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-phylo Searchable archive at http://www.mail-archive.com/r-sig-phylo@r-project.org/
