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Re: st: Meta-analysis of SMRs

From   Roger Harbord <>
To, "Fenty, J." <>
Subject   Re: st: Meta-analysis of SMRs
Date   Thu, 02 Sep 2004 09:59:30 +0100

The usual fudge when meta-analysing 2x2 tables is to add 0.5 to all cells of a study when any cell contains zero. So analogously for SMRs you could replace observed values of zero by 0.5. This is probably reasonable unless many of your studies have no or very few observed events. What to do in that case is not immmediately obvious to me - it might be possible to fit a suitable random-effects poisson model with an offset using -xtpoisson- or -gllamm- but don't ask me for the syntax!

Roger Harbord
Department of Social Medicine, University of Bristol

--On 01 September 2004 12:41 +0100 "Fenty, J." <> wrote:

Dear statlist

I am conducting meta-analyses of SMRs using the -meta- command which works
fine until you have observed values of zero. I am using the formulae

			log(SMR) = log(observed/expected)
			se(log(SMR)) = 1/sqrt(observed)

and both are undefined when the observed value=0. Does anyone have any
suggestions for dealing with this situation other than omitting studies with
zeroes or combining them with other studies? Are there other formulae to
calculate se(log(SMR)) which is not based solely on the observed value?

Many thanks,

Justin Fenty

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