odds ratio

Logistic Regression Models: Reversed odds ratios in SAS Proc Logistic–Use ‘Descending’

March 18th, 2009 by

If you’ve ever been puzzled by odds ratios in a logistic regression that seem backward, stop banging your head on the desk.

Odds are (pun intended) you ran your analysis in SAS Proc Logistic.

Proc logistic has a strange (I couldn’t say odd again) little default.  If your dependent variable Y is coded 0 and 1, SAS will model the probability of Y=0.  Most of us are trying to model the probability that Y=1.  So, yes, your results ARE backward, but only because SAS is testing a hypothesis opposite yours.

Luckily, SAS made the solution easy.  Simply add the ‘Descending’ option right in the proc logisitic command line.  For example:

MODEL Y = X1 X2;

All of your parameter estimates (B) will reverse signs, although p-values will not be affected.
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