[R] GLM function with poisson distribution

[BXC (Bendix Carstensen)]

> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch 
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Florian Menzel
> Sent: Tuesday, January 25, 2005 3:22 PM
> To: r-help at stat.math.ethz.ch; r-help at stat.math.ethz.ch
> Subject: [R] GLM function with poisson distribution
> 
> 
> Hello all, 
> I found a weird result of the GLM function that seems
> to be a bug.
> The code:
>   a=c(rep(1,8),rep(2,8))
>   b=c(rep(0,8),rep(3,8))
>   cbind(a,b)
>   model=glm(b~a, family=poisson)
>   summary(model)

It' because one of the values of b is 0, hence the linear predictor for
the corresponding level of a is -Inf, viz. the value -49 for the
intercept,
and the huge standard errors. Usual theory breaks down.

Replace the 0s with 1s and you get something which is closer
to what is covered by standard theory.

Bendix Carstensen

> generates a dataset with two groups. One group
> consists entirely of zeros, the other of 3's (as
> happened in a dataset I'm analyzing right now). Since
> they are count data, one should apply a poisson
> distribution. A GLM with poisson distribution delivers
> a p value > 0.99, thus, completely fails to detect the 
> difference between the two groups. Why not and what should I 
> do to avoid this error? A quasipoisson distribution detects 
> the difference but I'm not sure whether it's appropriate to 
> use it. Thanks a lot to everybody who answers!
> 		 Florian
> 
> Version information:
> version 1.9.0 (2004-4-12)
> os mingw32
> arch i386
> 
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