[R] negative predicted values in poisson glm

[Thomas Lumley]

On Wed, 18 Jan 2006, P. Olsson wrote:

> Dear  R  helpers,
> running the following code of a glm model of the family poisson, gives
> predicted values < 0. Why?

Look at the help page for predict.glm, particularly the "type" argument. 
Then exponentiate the results.

 	-thomas


>
> library(MASS)
> library(stats)
> library(mvtnorm)
> library(pscl)
> data(bioChemists)
> poisson_glm <-  glm(art ~ fem + mar + kid5 + phd + ment, data = bioChemists,
> family = poisson)
> predicted.values = predict(poisson_glm)
> range(predicted.values)
>
>
> Thank you in advance for any hints.
> Best regards,
> P. Olsson
>
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Thomas Lumley			Assoc. Professor, Biostatistics
tlumley at u.washington.edu	University of Washington, Seattle