Date: Thu, 25 Oct 2007 10:17:54 -0500
Reply-To: Mary <firstname.lastname@example.org>
Sender: "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From: Mary <mlhoward@AVALON.NET>
Subject: Logistic regression when I have only a small number of people in
the event category.
Content-Type: text/plain; charset="iso-8859-1"
I have a question about Logistic Regression when I have a small number of people in the event category.
I'm running a model looking at whether people have the disease or not based on the value of a Genetic SNP, which is a categorical variable coded 0, 1, 2, and the base reference is 0. Thus, in a simple model, I'm doing this:
proc logistic data=set1;
class snp(param=ref ref='0');
On one of the diseases that I'm looking at, I've got a very small n size for the number of people that have the disease (n=19), and I have 300 controls.
My question is, if I get a significant Likelihood ratio for the overall model, but then when I look at the individual odds ratios (1 vs 0, 2 vs 0), if those individual odds ratios are not significant, can I still rely on the overall likelihood ratio or is that not valid as well? On some of the odds ratios I do get SAS's message that the validity of the model is questionable, but on others I just get the odds ratios whose confidence intervals include 1 and thus are not significant, but the overall likelihood ratio is significant.
If I can't use logistic regression in looking at a disease in which very few people have the disease, might there be another alternative?