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

by Karen

FacebookTwitterGoogle+Share

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:

PROC LOGISTIC DESCENDING;
MODEL Y = X1 X2;
RUN;

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

Logistic Regression WorkshopLearn how to recognize the need for, conduct, evaluate, and interpret a logistic regression model and to use it to predict outcome probabilities. Check out our ~8-hour online workshop Logistic Regression for Binary, Ordinal, and Multinomial Outcomes.
 

{ 1 comment… read it below or add one }

Jim Hess July 11, 2016 at 1:57 pm

In some fields it has been common to code Yes/No responses as Yes=1 No=2.

Reply

Leave a Comment

Please note that Karen receives hundreds of comments at The Analysis Factor website each week. Since Karen is also busy teaching workshops, consulting with clients, and running a membership program, she seldom has time to respond to these comments anymore. If you have a question to which you need a timely response, please check out our low-cost monthly membership program, or sign-up for a quick question consultation.

Previous post:

Next post: