[R] Odds Ratios in rms package

[David Winsemius]

On Jun 20, 2012, at 12:12 PM, Sebastian Pölsterl wrote:

> Hi,
>
> I'm using the rms package to do regression analysis using the lrm
> function. Retrieving odds ratios is possible using summary.rms.  
> However,
> I could not find any information on how exactly the odds ratios for
> continuous variables are calculated. It doesn't appear to be the odds
> ratio at 1 unit increase, because the output of summary.rms did not
> match the coefficient's value.
>
> E.g. print gives me:
>
>                Coef    S.E.   Wald Z Pr(>|Z|)
> age              0.1166 0.0289  4.04  <0.0001
>
> whereas summary gives me:
>
> Factor      Low     High     Diff.   Effect S.E. Lower 0.95 Upper 0.95
> age         27.0000 37.00000 10.0000  0.78  0.20  0.40        1.17
> Odds Ratio 27.0000 37.00000 10.0000  2.19    NA  1.49        3.22
>
> Does anybody know how these values are obtained, especially in the
> presence of interactions?

It is explained in the first paragraph of ?summary.rms, :

" By default, inter-quartile range effects (odds ratios, hazards  
ratios, etc.) are printed for continuous factors,"

... and the labeling makes it fairly clear (at least it was for me)   
that it is an odds ratio for a change in predictor value from the 25th  
to the 75th percentile (which are the values in the Low and High  
columns)

In the presence of interactions you should not be looking at the  
coefficients, but rather at the predictions.

?Predict

-- 
David.


>
> Best regards,
> Sebastian
>
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David Winsemius, MD
West Hartford, CT