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Re: st: Convert Odds Ratios to Risk Ratios after clogit?


From   Marcel Raab <[email protected]>
To   [email protected]
Subject   Re: st: Convert Odds Ratios to Risk Ratios after clogit?
Date   Thu, 13 Mar 2014 15:50:15 +0100

Christian,
Being a social scientist I am not used to the terminology in epidemiology. After I checked the Stata Manual entry on -clogit- I would say that I have k1i:k2i matching with k1i >= 1 and i denoting that matching can change from group to group. My groups/strata are families consisting of a varying number of children (2-10). For most groups I have only one case but in roughly 25% of the groups I have multiple positive outcomes.

I was asking for risk ratios because our Information and Communication Department is struggling with the Odds Ratio interpretation. They (and also I) would prefer a more accessible interpretation of our multivariate results.

Therefore, I was first trying to work with some kind of predicted probabilities. But I had the impression that these are not correct in my case. The pc1-option of predict calculates the probability of a positive outcome conditional on one positive outcome within group but I have multiple positive outcomes in a lot of groups. And the pu0-option calculates the probability of a positive outcome, assuming that the fixed effect is zero which according to a Statalist-post of Maarten Buis is "a rather weird hybrid between average marginal effects and marginal effects at average values of explanatory variables" (http://www.stata.com/statalist/archive/2012-03/msg01167.html). Finally I turned to the OR to RR conversion which also seems to be problematic if I understand you correctly (unmatched matched case-control)..

Marcel



Am 13.03.2014 13:57, schrieb Christian Bautista:
Marcel,

The formula given by Zhang and Yu is for odds ratios from unmatched
case-control studies but I see that you're using "clogit" which is the
standard method for matched case-control studies. What kind of matching
you have used? frequency-matching or incidence density-matching?

/Christian

 > Date: Thu, 13 Mar 2014 13:48:58 +0100
 > From: [email protected]
 > To: [email protected]
 > Subject: st: Convert Odds Ratios to Risk Ratios after clogit?
 >
 > Dear Statalisters,
 >
 > although I am aware of the criticism that has been raised against
 > converting Odds Ratios to Risk Ratios I was wondering if the formula
 > proposed by Zhang and Yu (1998) can also be used in the context of a
 > fixed effects model. As the -oddsrisk- module does not work for this
 > purpose I was trying to apply the formula manually
 >
 > RR = OR / ((1 - pu) + (pu * OR))
 > (pu = incidence rate of the unexposed group)
 >
 > Here is my example:
 >
 > . webuse union, clear
 > . clogit union age grade not_smsa, group(idcode) or
 > . sum union if not_smsa == 0 & e(sample) // mean is pu(?)
 > . display exp(_b[not_smsa]) / ((1 - r(mean)) + (exp(_b[not_smsa]) *
 > r(mean)))
 >
 > In the example the OR = .9673623 and the RR = .97958943.
 >
 > I read about the conversion of ORs to RRs only recently and I am
 > definitely not an expert in the field of non linear models. Hence, I
 > would be very glad if anyone could help me with this issue.
 > Is it appropriate to convert ORs in RRs in a clogit context like
 > suggested above?
 > Is there an alternative/superior method to do it? And finally, if it is
 > possible what would be the best way to obtain confidence intervals for
 > the RRs?
 >
 > Thanks for your consideration,
 > Marcel
 >
 > --
 > Reference:
 > J. Zhang and K. Yu, 1998. What's the Relative Risk, JAMA, Vol 280, No
 > 19, pp 1690-1691.
 > *
 > * For searches and help try:
 > * http://www.stata.com/help.cgi?search
 > * http://www.stata.com/support/faqs/resources/statalist-faq/
 > * http://www.ats.ucla.edu/stat/stata/
*
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