'ODDSRISK': module to convert Logistic Odds Ratios to Risk Ratios now on SSC
I thought I had posted a notice about this module to the list yesterday, but
it seems that it did
not make it. I take the Digest, so do not see what is posted until the next
day. I apologize if the earlier one is posted today, as well as this one.
Zhang and Yu (1998, JAMA) proposed an algorithm to convert binary logistic
regression odds ratios to estimated risk ratios. I have seen the method used
in a number of medical research articles, and had written about the method in
a book I am writing. I noticed a couple of Statalist communications during
the past couple of weeks related to the relationship of odds ratio to risk
ratio. I wrote about this relationship in my "Negative Binomial Regression", but
not about this method and its rationale. Anyhow, I thought it might be helpful
to those interested in using this method to release a Stata implementation
now rather than with the forthcoming book several months from now. I wrote the
program in such a manner that the logistic odds ratio, estimated risk ratio,
and 95% confidence intervals of the estimated risk ratios are displayed for
each predictor in the model. The incidence rate of the unexposed risk factor,
or primary predictor of interest (binary (1/0)), which is required for
calculation, is also displayed.
The estimated risk ratios are particularly important when we wish to use
terms such as "likely" in place of "odds of" when interpreting the odds ratio of
a logistic model. This is primarily the case with the odds ratio is under
0.5 or over 2.5 and the incidence rate is greater than 10%. To say something
like, "patients having an anterior infarct are 50% more likely to die within 48
hours of hospital admission than are patients having just sustained an
infarct at another primary site", is using risk language. Such language may be
justified for many logistic models, but not all. Using the estimated risk
values and CI's calculated by -oddsrisk- presumably enhance the justification for
using risk language with logistic models when the conditions specified above
obtain.
Kit Baum has graciously posted the program, called -oddsrisk- and the
associated help file to the SSC site. You can download it by typing
. ssc install oddsrisk
on the Stata command line.
I have pasted the SSC site module description under my name below.
Best, Joseph Hilbe
DESCRIPTION/AUTHOR(S)
oddsrisk converts logistic regression odds ratios to relative
risk ratios. When the incidence of an outcome is common in the
study population; i.e. greater than 10%, the logistic regression
odds ratio no longer approximates the risk ratio. As the
incidence rate becomes more frequent, the more the odds ratio
overestimates the risk ratio when its value is greater than 1,
and the more it underestimates the risk ratio when under one. J.
Zhang and K. Yu proposed a method of adjusting the logistic
regression odds ratio in a cohort study or clinical trial so that
it approximates the risk ratio. This is particularly important
when the odds ratio is greater than 2.5 or under 0.5. The method
has also been shown to be applicable for retrospective and
observational studies as well.
Author: Joseph M. Hilbe
Arizona State University
[email protected] or [email protected]
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