[R-sig-ME] unbalanced data in nested lmer model

Jana Bürger jana.buerger at uni-rostock.de
Wed Mar 24 13:52:22 CET 2010

Dear List,

I have two questions regarding unbalanced data and lmer and could not 
find any hints in archives nor in my textbooks.

1) Is there a limit to the unbalance of case numbers in cells? Or would 
it be no problem if cell sizes vary between 0 and 53?

2) I have data of individual fields from a number of farms which are 
located in a number of regions. For 2 of 8 regions there is only 1 farm, 
the other regions have 2 farms. Which consequences does this fact have 
on the random effects part of the model summary if I choose the model
All y and x variables are field specific.

An anova(lm1, lm2) comparing with lm2<-lmer(y+x1+x2+...+(1|farm)) said 
models did not differ significantly and AIC was about the same. So I 
know there is no additional explanatory power including the region term.

Yet, I would like to keep the region effect in the model to separate and 
compare the effect size of region vs. farm. Is it valid to do so even if 
  some of the regions are only represented by one farm?

I would appreciate any thoughts on this very much.

Jana Bürger

Universität Rostock
Agrar-  und Umweltwissenschaftliche Fakultät
FG Phytomedizin
Satower Straße 48
18059 Rostock

Tel. 0381-498 31 71
Fax.0381-498 31 62

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