Re: [R] Negative intercept in glm poisson model

From: Douglas Bates <>
Date: Wed 02 Mar 2005 - 01:07:11 EST

Peter Dalgaard wrote:
> Douglas Bates <> writes:

>>Federico Gherardini wrote:
>>>I'm trying to fit a glm model using family=poisson(link =
>>>"identity"). The problem is that the glm function fits a model with
>>>a negative intercept, which sounds like a nonsense to me, being the
>>>response a Poisson variable.
>>Not really.  The negative intercept is on the scale of the linear
>>predictor.  The expected response, which is the exponential of the
>>linear predictor, is always positive.

> Not when the link is "identity"...! However, in that case, sufficient
> extrapolation will always lead to negative values, unless the slope is
> exactly zero. Whether a negative value is a problem for the intercept
> depends on whether it corresponds to a relevant extrapolation of data.

Yes, of course. Must learn not to post to a public list before the second cup of coffee. mailing list PLEASE do read the posting guide! Received on Wed Mar 02 01:26:00 2005

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