[R-sig-ME] Question regarding GLMMs and proportional data

Rachael Eaton eatonrac at msu.edu
Wed Jan 21 20:58:45 CET 2015


I am having some trouble identifying the best model structure for my data.
I am trying to construct models to look at a response variable of
proportions. These data are highly skewed toward zero, with a max value of
0.2. I also have one random variable and a few fixed effects. Therefore, I
decided to use GLMM. I applied a log-transformation of (1 + the response
variable). I then modeled these using glmer with a gamma family with an
inverse link.

Following concerns from a colleague, I also tried modeling the
untransformed data with a binomial family, since I know the denominators
for my proportion data. However, the model fit for this approach was not as
strong as for the gamma approach. AIC strongly favored the model using the
gamma distribution over the model using the binomial distribution.

If you have any recommendations or suggestions on how best to model these
data I've described, I would greatly appreciate it.

*Rachael Eaton*
PhD Candidate
Department of Zoology
Graduate Program in Ecology, Evol Biology & Behavior
Michigan State University
rachaeleaton.weebly.com <http://www.rachaeleaton.weebly.com>

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