[R] basic question predict GLM offset

David Winsemius dwinsemius at comcast.net
Sun Apr 15 01:37:00 CEST 2012

On Apr 14, 2012, at 6:47 PM, smfa wrote:

> Hi,
> I know this is probably a basic question... But I don't seem to find  
> the
> answer.
> I'm fitting a GLM with a Poisson family, and then tried to get a  
> look at the
> predictions, however the offset does seem to be taken into  
> consideration:
> model_glm=glm(cases~rhs(data$year,2003)+lhs(data$year,2003),
> offset=(log(population)), data=data, subset=28:36, family=poisson())
> predict (model_glm, type="response")
> I get cases not rates...
> I've tried also
> model_glm=glm(cases~rhs(data$year,2003)+lhs(data$year,2003)+
> offset(log(population)), data=data, subset=28:36, family=poisson())
> with the same results. However when I predict from GAM, using mgcv,  
> the
> predictions consider the offset (I get rates).

The beta coefficients are the log-rate-estimates when you use  
log(population) as the offset.

> I'm missing something?

You are most definitely missing the part where you include 'data'.

David Winsemius, MD
West Hartford, CT

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