Programming the statistical procedures from SAS

Incidence Risk Ratio (IRR)

New Contributor
Posts: 4

Incidence Risk Ratio (IRR)

Hi everyone

I am trying to get the incience risk ratio (irr) for each of my independent variables while also using the zero inflated negative binomial regression. How can i do this as the output for ZINB does not give the irr.


Super Contributor
Posts: 287

Re: Incidence Risk Ratio (IRR)

In my opinion, you should forget this idea and fit data with Poisson regression.

This is because the likelihood obtained from observing person time and events is the same function of the parameters as when you have poisson distributed counts. Therefore, Poisson regression is just used as a trick to maximize the likehood function and thereby find the right estimates of the IRR. It doesnt matter that the fitstatistics shows a misfit, as the data was not Poisson distributed anyway.

Negative binomial regression will only result in biased standard errors.

SAS Employee
Posts: 228

Re: Incidence Risk Ratio (IRR)

A negative binomial or ZINB model is for a count response.  A risk ratio is a ratio of event probabilities, so that assumes that the response is binary, not a count.  Assuming your response is really a count, then you might want to estimate a RATE ratio. This note discusses estimating rates and rate ratios with a count model. This note talks about estimating rates using zero-inflated count models. 

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