[R] Poisson GLM with a logged dependent variable...just asking for trouble?

Mark Na mtb954 at gmail.com
Fri Jul 1 23:09:39 CEST 2011

Dear R-helpers,

I'm using a GLM with poisson errors to model integer count data as a
function of one non-integer covariate.

The model formula is: log(DV) ~ glm(log(IV,10),family=poisson).

I'm getting a warning because the logged DV is no longer an integer.

I have three questions:

1) Can I ignore the warning, or is logging the DV (resulting in
non-integers) a serious violation of the Poisson error structure?

2) If the answer to #1 is "no, don't ignore it, it's serious" then can
I use a quasipoisson error structure instead (does not  give the same
warning) and if so are there any pitfalls to using the quasipoisson
model? Are there any better alternatives for count data where the
counts must be logged? Or, should I just abandon logging the DV? In
that case, how could I compare the fit of a Poisson model (without
logging the DV) to that of a GLM with normal errors (with a logged
DV). AIC would not be valid because the DVs are different, right?

3) The quasipoisson model doesn't return an AIC value. Why, and is
there anything I can do to calculate AIC manually, that would allow me
to compare this model to other models?

Many thanks in advance for your help!

Cheers, Mark

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