Dear mixed-model brain trust: 

 

I am comparing snag (dead tree) densities 1 year and 5 years after
silvicultural treatment in forest plots to densities prior to treatment. In
nlme, my model is 

 

lme(snagnum~treatment, random=(~1|plot), correlation=corExp(form=~year)). 

 

{Treatment is a factor with values of Pre/1-year post/5-year post}. This
gives reasonable output, but I'm having a niggling doubt that I should be
using something akin to a negative binomial distribution, since about half
of the values are zeros (i.e., many plots had no snags prior to treatment,
and did not gain additional snags as a result of treatment). Can anyone
suggest an appropriate package and associated syntax for doing this mixed
model based on an alternative probability density function?

 

--Seth W. Bigelow


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