Date:         Mon, 10 May 2004 15:24:55 -0400
Reply-To:     Peter Flom <flom@NDRI.ORG>
Sender:       "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From:         Peter Flom <flom@NDRI.ORG>
Subject:      Huge confidence interval in logistic regression
Comments: To: STAT-L@LISTS.MCGILL.CA
Content-Type: text/plain; charset=US-ASCII

Hello

Apologies for cross-posting, but I have such good responses from both
lists.....

We have data on 96 large metropolitan areas in the USA

The dependent variable is presence or absence of a syringe exchage; IVs
include percent MSM in the city, presence of an ACTUP chapter in the
city, and percent college educated  (these were gotten by a stepwise
procedure, and YES, I know that's a bad idea, but this is not analysis
for my own project, and higher-ups insist.....)

For percent MSM, we are getting a huge confidence interval on the odds
ratio, something like 1.5  to 150.  I thought this might be due to
outliers, we removed a couple, and it narrowed the CI only slightly.  I
thought it might be due to collinearity, but the largest condition index
is only 14.

Any other ideas?

Thanks in advance, as always

Peter