Relative Risk in logistic regression

I am curious why one would want risk ratios.  Unlike odds ratios, they are
not interpretable without reference to the base risk.  For example a risk
ratio of 2 cannot possibly apply to anyone with a starting risk exceeding
1/2.

I think it is most helpful to use one of the existing nomograms to show
someone how the base risk and odds ratio translate to final risk, for a
range of base risk.

Frank

David Winsemius wrote

On Jan 30, 2013, at 5:49 AM, aminreza Aamini wrote:

Hi all,
I am very grateful to all those who write to me
1) how i  can  obtain relative risk (risk ratio) in logistic  
regression in R.
2) how to obtain  the predicted risk for a certain individual using
fitted regression model in R.

You obtain the predicted probabilities with something like:

predict(model, data.frame(x1="a", x2=30), type = "response")

See ?predict.glm

This would give the odds ratios (similar but larger than the risk  
ratios):

exp(coef(model))

-- 
David Winsemius, MD
Alameda, CA, USA

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Frank Harrell
Department of Biostatistics, Vanderbilt University
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