[R-sig-ME] Minimum number of levels for mixed model

nrm2010 nrm2010 at zoho.com
Wed Feb 6 18:34:57 CET 2013

Dear Mixed Modelers:

The oft-mentioned glmm wiki FAQ recommends a minimum of 5 or 6 levels
 when using a mixed model. I am not sure how to interpret this statement.  
I have 4 experimental treatments, each with 3 replicates, for a total of 12 plots.
  Measurements were made over 4 years and there are thousands of individual
 data points within each of the 12 plots.  I was planning to use nested random
 effects of plot within treatment within year. So I would have 3 replicate plots 
within each of 4 treatments within each of 4 years. 

My questions:
For nested effects, at what level does the recommendation pertain? Is it only
the lowest level?  Would 5 or 6 plots within 3 treatments within 3 years be acceptable?
Or should one have 5 or 6 plots within 5 or 6 treatments within 5 or 6 years? Is the
amount of data within plot irrelevant to this issue? 

If 4 treatments are insufficient for a mixed model, is treatment as fixed effect
the only alternative?  I need an approach for which there is a method for multiple
comparison of means.  I believe lm, lme, lme4 are the only options since the aov 
methods do not accept an Error term.

Thank you for your thoughts.

Toby Gass

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