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@(protected)
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.