[R] How to compute an exact Poisson model with R?

[David Winsemius]

Sent from my iPhone

> On Aug 22, 2026, at 4:38 PM, David Winsemius <dwinsemius using comcast.net> wrote:
> 
> The point estimates for regression should be the same although the CI's for the MCMC methods might be wider (more conservative if that is what you mean by "more accurate"). I

I was misinterpreting the summary output of mcexact. The glm coefficients are not supposed to be estimates. The function is only doing a gof test. 

David Winsemius


> 
> I was able to install Brian Caffo's archived 'exactLoglinTest' package for my MacOS system with R 4.6.1 after adding:
> 
> 
> CPPFLAGS += -I/Library/Frameworks/R.framework/Versions/4.5-arm64/Resources/include
> 
> to my .R/Makevars file. That allows the compilation step to find PrtUtil.h from an earlier version of R since it has been folded into R 4.6.x rather than currently being exposed.
> 
> The Notes that Caffo chose not to fix did not cause any errors, at least to my minimal testing with the package's examples.
> 
> Best;
> 
> David Winsemius, MD, MPH
> 
>> On Aug 21, 2026, at 1:21 AM, iagogv via R-help <r-help using r-project.org> wrote:
>> 
>> Hi all,
>> 
>> Can an exact Poisson regression be computed with R, instead of using the
>> standard (glm) Poisson using maximum likelihood?
>> 
>> I am working with a collegue in a project, and I'd like to reproduce his
>> method, if not his results. He uses Stata, particularly the expoisson
>> function (https://www.stata.com/help.cgi?expoisson) in order to compute
>> the exact Poisson regression, relevant for small samples.
>> 
>> expoisson fits an exact Poisson regression model, which produces more
>> accurate inference in small samples than standard
>> maximum-likelihood-based Poisson regression
>> 
>> I know about exact Poisson tests
>> (https://stat.ethz.ch/R-manual/R-devel/RHOME/library/stats/html/poisson.test.html),
>> but it does not allow to include offset.
>> 
>> So, is there some way to reproduce exact Poisson regression with R?
>> 
>> Thanks!
>> 
>> Best,
>> 
>> --
>> Iago
>> [[alternative HTML version deleted]]
>> 
>> ______________________________________________
>> R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide https://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
> 
> ______________________________________________
> R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide https://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.