Glitch in calculating relative risks using xtgee

[R Ratnayake]

Hello all,

I am having a problem calculating relative risks using the xtgee command (to account for clustering of variation owing to repeated measurements within a set of health facilities).

The command I am entering looks something like this, where _exposure_ indicates the time period (1=follow-up, 0=baseline) and _direct_ indicates the behavior (1=correct, 0=incorrect):

. xtgee direct exposure, i(group2) fam(bin) link(log) corr(exc) eform

What I get is a different relative risk than if I used:

. cs direct exposure

I suspect that xtgee us using the _exposure_ variable as the dependent variable. I'm not sure though. Is a conversion to a risk ratio not happening?

Apologies in advance if this is a simple matter that I'm missing.

Thanks.

[Rich Goldstein]

cs gives risk ratios while your xtgee command gives odds ratios - those are different things

[R Ratnayake]

That is very true, I should've looked at the manual. Thanks Rich.

[Joseph Coveney]

xtgee is not using the exposure variable as the dependent variable. And xtgee . . ., family(binomial) link(log) eform does give risk ratio (in the lines before the comment in the output below, compare its point estimate with those for cs and glm; glm even labels the coefficient "Risk Ratio").

The difference is probably due to the working correlation structure. Try using corr(independent) and see whether the marginal risk ratio estimate then matches that of cs.

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------------------------------------------------------------------------------
ÿÿÿÿÿÿÿÿÿÿÿÿÿ|ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿRobust
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-------------+----------------------------------------------------------------
ÿÿ1.exposureÿ|ÿÿÿÿÿÿÿ23.4ÿÿÿ25.15352ÿÿÿÿÿ2.93ÿÿÿ0.003ÿÿÿÿÿ2.845954ÿÿÿÿ192.3995
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------------------------------------------------------------------------------

.ÿxtgeeÿforeignÿi.exposure,ÿi(rep78)ÿfamily(binomial)ÿlink(log)ÿcorr(independent)ÿvce(robust)ÿeformÿnolog

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Correlation:ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿindependentÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿavgÿ=ÿÿÿÿÿÿÿ12.3
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Scaleÿparameter:ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ1ÿÿÿÿÿÿProbÿ>ÿchi2ÿÿÿÿÿÿÿ=ÿÿÿÿÿ0.0034

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------------------------------------------------------------------------------
ÿÿÿÿÿÿÿÿÿÿÿÿÿ|ÿÿÿÿÿÿÿÿÿÿÿÿÿSemirobust
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-------------+----------------------------------------------------------------
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------------------------------------------------------------------------------

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Groupÿvariable:ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿrep78ÿÿÿÿÿÿNumberÿofÿgroupsÿÿ=ÿÿÿÿÿÿÿÿÿÿ6
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Correlation:ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿexchangeableÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿavgÿ=ÿÿÿÿÿÿÿ12.3
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-------------+----------------------------------------------------------------
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------------------------------------------------------------------------------

.ÿxtgeeÿforeignÿi.exposure,ÿi(rep78)ÿfamily(binomial)ÿlink(log)ÿcorr(exchangeable)ÿvce(robust)ÿeformÿnolog

GEEÿpopulation-averagedÿmodelÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿNumberÿofÿobsÿÿÿÿÿ=ÿÿÿÿÿÿÿÿÿ74
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Link:ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿlogÿÿÿÿÿÿObsÿperÿgroup:
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Scaleÿparameter:ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ1ÿÿÿÿÿÿProbÿ>ÿchi2ÿÿÿÿÿÿÿ=ÿÿÿÿÿ0.0328

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------------------------------------------------------------------------------
ÿÿÿÿÿÿÿÿÿÿÿÿÿ|ÿÿÿÿÿÿÿÿÿÿÿÿÿSemirobust
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-------------+----------------------------------------------------------------
ÿÿ1.exposureÿ|ÿÿÿ11.51268ÿÿÿ13.17796ÿÿÿÿÿ2.13ÿÿÿ0.033ÿÿÿÿÿ1.221378ÿÿÿÿ108.5182
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------------------------------------------------------------------------------

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