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

[ONKELINX, Thierry]

Dear Mark,

I think you want glm(DV ~ log10(IV), family=poisson)
Note that the poisson family uses the log-link by default. Hence you don't need to log-transform DV yourself.

Best regards,

Thierry

----------------------------------------------------------------------------
ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek
team Biometrie & Kwaliteitszorg
Gaverstraat 4
9500 Geraardsbergen
Belgium

Research Institute for Nature and Forest
team Biometrics & Quality Assurance
Gaverstraat 4
9500 Geraardsbergen
Belgium

tel. + 32 54/436 185
Thierry.Onkelinx op inbo.be
www.inbo.be

To call in the statistician after the experiment is done may be no more than asking him to perform a post-mortem examination: he may be able to say what the experiment died of.
~ Sir Ronald Aylmer Fisher

The plural of anecdote is not data.
~ Roger Brinner

The combination of some data and an aching desire for an answer does not ensure that a reasonable answer can be extracted from a given body of data.
~ John Tukey

> -----Oorspronkelijk bericht-----
> Van: r-help-bounces op r-project.org [mailto:r-help-bounces op r-project.org]
> Namens Mark Na
> Verzonden: vrijdag 1 juli 2011 23:10
> Aan: r-help op r-project.org
> Onderwerp: [R] Poisson GLM with a logged dependent variable...just asking for
> trouble?
> 
> 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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