[R-sig-ME] Zero random effect variance?

[Dave Marvin]

For the following dataset (described at the bottom of this email), a boxplot of the data by chamber

> height=read.table("height.txt",header=TRUE)
> boxplot(HtChg~Chamber,data=height)

shows there is clearly a lot of chamber-to-chamber variation in the response variable. However, if I run a random intercept-only model:

> lmer(HtChg~1+(1|Chamber),data=height)

I get 0 variance for the random intercept. Same is true if I then include any categorical fixed effects. Does this seem correct, and if so why? 
-Dave


> I am analyzing the growth response (Height Change) of two plant types (vines vs. trees) to different CO2 levels, for a mix of species of each plant type in plant growth chambers (Chamber). CO2 and FT are categorical predictors, each with two levels (elevated/ambient CO2, vine/tree plant Functional Types). Each growth chamber had the same mix of 8 species (Spp).