st: Convert Odds Ratios to Risk Ratios after clogit?

[Marcel Raab <marcel.raab@wzb.eu>]

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.
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