[R-sig-ME] Binomial GLMM vs GLM question
Justin Touchon
jtouchon at bu.edu
Thu May 15 23:40:08 CEST 2008
Dear Dr. Bates and other LMER experts,
I am admittedly entry level in my R and mixed-model knowledge, but
I'm hoping that someone can help me and also forgive my lack of
insight. Over 3 years, I monitored survival of 350 egg masses at two
ponds. I thus have one continuous variable (rainfall) and two discrete
variables (year and pond). My response variable, mortality, is coded as
a two column matrix featuring eggs survived and eggs dead. I'm primarily
interested in the effect of rain on survival, but also if rain has
different impacts at the different ponds and how much survival varied
over the three years. Originally, I though I could tackle this with a
binomial GLM, but do I need a binomial GLMM instead, as rainfall and
year would be random and pond fixed? The problem with this is trying to
make biological sense out of the results. I've spent the last week
reading all the past posts about why p-values can't be calculated and
all that, which I'm fine with. But what can I say about the effects of
rainfall or year on egg survival from the variance estimates? Also,
doesn't LMER require that random factors be normally distributed,
because my rainfall measurements are far from it. Is that a problem?
Thank you in advance for any advice you can give.
-Justin Touchon
My model and output are as follows:
> LMER.1<-lmer(mort~Pond + (Pond|total_rainfall) + (1|Year),
family=binomial, data= FieldData0305)
> summary(LMER.1)
Generalized linear mixed model fit using Laplace
Formula: mort ~ Pond + (Pond | total_rainfall) + (1 | Year)
Data: FieldData0305
Family: binomial(logit link)
AIC BIC logLik deviance
7657 7680 -3822 7645
Random effects:
Groups Name Variance Std.Dev. Corr
total_rainfall (Intercept) 21.66535 4.65461
Pond[T.Ocelot] 6.44297 2.53830 -0.627
Year (Intercept) 0.74082 0.86071
number of obs: 350, groups: total_rainfall, 48; Year, 3
Estimated scale (compare to 1 ) 4.603433
Fixed effects:
Estimate Std. Error z value Pr(>|z|)
(Intercept) -0.4678 1.0173 -0.4598 0.646
Pond[T.Ocelot] -0.9831 0.9330 -1.0538 0.292
Correlation of Fixed Effects:
(Intr)
Pnd[T.Oclt] -0.648
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