Dear List Members,

I am using a mixed-effects model in lme and would like to know whether I am
using the proper structure for the random-effects component of the model. My
experiment consists of three categorical treatments (fire, water, seed)
arranged in a split-plot design. The fire treatment (2 levels) and water
treatment (3 levels) were assigned to plots, and the seed treatment (2
levels) was assigned to two subplots within each plot. There are 60 total
plots (120 total subplots), divided equally among two large blocks (30 plots
per block). I measured species richness in each subplot in three separate
years. My goal is to test for main effects of the three treatments, a main
effect of year, and all interactions. My current model consists of four
factorial fixed effects (fire, water, seed, year) and 1 random effect
(block), with plots nested within blocks (to account for the split-plot
structure of the experiment):

model = lme(species.richness ~ water*fuel*seed*year, random = ~1 |
block/plot)

The ANOVA output includes two denominator degrees of freedom (denDF): 53
denDF for plot factors (fire, water, fire x water interaction) and 270 denDF
for split-plot factors (everything else).

I would greatly appreciate feedback as to whether the random-effects
component of the model looks appropriate, and if not, how it should be
modified.

Thanks very much!

Cheers,

Jonathan


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Jonathan A. Myers
Department of Biological Sciences
Division of Systematics, Ecology, and Evolution
Louisiana State University
Baton Rouge, LA 70803 USA

E-mail: jmyer19@lsu.edu
Telephone: 225-578-7567
Fax: 225-578-2597

Website: http://www.biology.lsu.edu/labpages/harmslab/jmyers/index.html
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