[R] R-help Digest, Vol 282, Issue 3

[Ravi Varadhan]

To my knowledge, there is no built‑in function in R that exactly replicates Stata’s expoisson. The main issue is that it can be computationally heavy since it conditions on the sum of total counts (conditional MLE).  It can only be practical in small samples. We should be able to get similar estimates using Firth's bias correction, for example, although I myself have not tested this claim.   The brglm2 package implements Firth's bias reduction for GLMs.
Ravi
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Subject: R-help Digest, Vol 282, Issue 3


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Today's Topics:

   1. How to compute an exact Poisson model with R? (iagogv)

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Message: 1
Date: Fri, 21 Aug 2026 10:21:14 +0200
From: iagogv <iagogv using disroot.org>
To: R Help <r-help using r-project.org>
Subject: [R] How to compute an exact Poisson model with R?
Message-ID: <15fca7e36128374222c8307bebf36c1e using disroot.org>
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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<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<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
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