R help - Sep 2011 - How to do Multiple Comparisons for a Mixed Effects Model

Hello everyone I am currently trying to conduct analysis of my graduate thesis
data using a mixed effects model and I have reached an impass. When I try to
conduct a multiple comparison, I get an error (See below): > fm3<-
lme(abovegroundbiomass.m.2~medium*amelioration*fertilizer*treatment,
random=~1|block/medium/amelioration/fertilizer) > tukeytest<-glht(fm3,
linfct=mcp(treatment="Tukey"))
Error in contrMat(table(mf[[nm]]), type = types[pm]) : 
  less than two groups
 

I wonder if anyone could help me interpret this error. Many thanks. Allan Carson

 		 	   		  
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Allan Carson <acarson <at> unbc.ca> writes:

[snip]
> When I try to conduct a 
> multiple comparison, I get an error (See below):
> fm3<-
lme(abovegroundbiomass.m.2~medium*amelioration*fertilizer*treatment,
>  random=~1|block/medium/amelioration/fertilizer) 
> tukeytest<-glht(fm3, linfct=mcp(treatment="Tukey"))
> Error in contrMat(table(mf[[nm]]), type = types[pm]) : 
>   less than two groups

  You should probably re-post this question to the r-sig-mixed-models
mailing list, which specializes (as you would guess) in mixed models.
 
  I'm a little suspicious of your random effects specification.  Are there
really random effects at all four levels?  i.e., are there four separate
hierarchical levels at which the predictors vary?

  It's hard to diagnose this problem without a reproducible example.
Is your model fit otherwise sensible?