Dear lmer-ers, My thanks for all of you who are sharing your trials and tribulations publicly.
I was hoping to elicit some feedback on my thoughts on denominator degrees of freedom for F ratios in mixed models. These thoughts and practices result from my reading of previous postings by Doug Bates and others. - I start by assuming that the appropriate denominator degrees lies between n - p and and n - q, where n=number of observations, p=number of fixed effects (rank of model matrix X), and q=rank of Z:X. - I then conclude that good estimates of P values on the F ratios lie between 1 - pf(F.ratio, numDF, n-p) and 1 - pf(F.ratio, numDF, n-q). - I further surmise that the latter of these (1 - pf(F.ratio, numDF, n-q)) is the more conservative estimate. When I use these criteria and compare my "ANOVA" table to the results of analysis of Helmert contrasts using MCMC sample with highest posterior density intervals, I find that my conclusions (e.g. factor A, with three levels, has a "significant effect" on the response variable) are qualitatively the same. Comments? Hank Dr. Hank Stevens, Assistant Professor 338 Pearson Hall Botany Department Miami University Oxford, OH 45056 Office: (513) 529-4206 Lab: (513) 529-4262 FAX: (513) 529-4243 http://www.cas.muohio.edu/~stevenmh/ http://www.muohio.edu/ecology/ http://www.muohio.edu/botany/ "E Pluribus Unum" ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
