[R-sig-ME] help with repeated measures on a split-plot experiment

[Mike Dunbar]

Dear James 

Some quick initial comments.

A. You are probably trying to make a much too complex model. I think what you want is:

lmer(response ~ snow*warm*year (1|plot), cass))

As plot is the only random effect, all the others are fixed. 

B. Are you sure there is likely to be a measurable autoregressive structure to the time series data beyond that which is accounted for by the plot random effect. Just looking at the data listing, there are only three years (1995, 2000, 2007), or is this a sub-sample? With only three years, and these not being sequential years, you may be asking too much of your data. 

C. I might be missing something but are you really interested in year as a fixed effect? With this included, are there any degrees of freedom left for the residual error, you'll need to get your replication from somewhere. Be warned that lmer does seem to give results even when all dfs are used up by fixed effects and their interactions (I'm not sure why), but you need to be able to judge that you have not fitted a sensible model.

cheers

Mike





>>> "James Hudson" <jmghudson at gmail.com> 28/05/2008 02:25:53 >>>
I require assistance to properly code an lmer function. I have repeatedly
measured vegetation in a split-plot, agricultural-style experiment. A pdf of
the dat file is attached. If there is any additional information I should
include, please let me know.

The design follows:
Plots were assigned to 1 of 3 snow levels (1, 2 or 3). Then, each plot was
split into 1 of 2 temperature levels (A or B). These split-plots were
replicated 6 times each. All plots were then measured repeatedly over
several years.

I am interested in the fixed effects of the treatments (snow, temperature,
time, and their interactions).

My statistical background is not advanced enough for me to properly code the
random effects of both repeated measures and the split-plot design. I expect
an AR(1) structure for 'year.' Here's my attempt at the code (it doesn't
work):


cass<-as.data.frame(read.csv("cass.csv", header=TRUE))
lmer(response ~ snow*warm*year + (1|snow/warm/year) + (0+year|plot), cass))

Any help would be appreciated.

James Hudson
UBC Geography


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