[R] Poisson Regression - Query

[Achim Zeileis]

On Sat, 19 Sep 2009, Axel Urbiz wrote:

> Hi All,
>
> My dependent variable is a ratio that takes a value of 0 (zero) for 95% of
> the observations and positive non-integer values for the other 5%. What
> model would be appropriate? I'm thinking of fitting a GLM with a Poisson ~.
> Now, becuase it takes non-integer values, using the glm function with
> Poisson family issues warning messages.

If it is a ratio of (integer-valued) counts in the numerator and some 
known denominator, say
   ratio = count/denom
then one standard approach would be to fit
   glm(count ~ ..., offset = log(denom), data = ..., family = poisson)
because this corresponds to
       log(count)       = x'beta + log(denom)
   <=> log(count/denom) = x'beta

To address the excess zeros, you could use a hurdle model or a 
zero-inflated model. See
   http://www.jstatsoft.org/v27/i08/

Best,
Z

> Warning messages:
> 1: In dpois(y, mu, log = TRUE) : non-integer x = 0.430783
> 2: In dpois(y, mu, log = TRUE) : non-integer x = 0.162519
> 3: In dpois(y, mu, log = TRUE) : non-integer x = 0.162519
> 4: In dpois(y, mu, log = TRUE) : non-integer x = 0.162519
> 5: In dpois(y, mu, log = TRUE) : non-integer x = 0.371564
>
> I'll appreciate your thoughts.
>
> Thanks!
>
> Axel.
>
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>
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