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st: RE: how to adjust for covariates?

From   "Visintainer PhD, Paul" <[email protected]>
To   "'[email protected]'" <[email protected]>
Subject   st: RE: how to adjust for covariates?
Date   Mon, 11 Jan 2010 09:56:10 -0500

In your first syntax:

xi: logistic <DV> i.snp sex ethnics

The model assumes that "ethnics" is an ordinal value.  The coefficient is giving you the change in the dependent variable, DV, for each unit change in ethnics.  This doesn't make any sense because "ethnics" is a 4-level categorical variable (nominal), not an ordinal variable.  Treat your ethnics variable the same way you treated your SNP variable.  

Your second syntax is the correct approach:

xi: logistic <DV> i.snp sex i.ethnics
Paul F. Visintainer, PhD
Epidemiology and Biostatistics Core
Baystate Medical Center

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of [email protected]
Sent: Monday, January 11, 2010 8:03 AM
Subject: st: how to adjust for covariates?

Hello list,
Date: Mon, 11 Jan 2010 13:49:52 +0100
Message-ID: <[email protected]>
From: [email protected]
To: undisclosed-recipients:;

I'm doing a SNP analysis using the xi:logistic function.
To exclude possible confounder, I'm adjusting my calculations for different variables, e.g. sex and ethnics.
My study population consists of 4 different ethnicities, coded as 1 2 3 4. sex is coded 1 2. SNPs are coded 1 2 3 (e.g. CC/CT/TT).
I don't understand the following:

When I calculate:

xi:logistic i.snp sex ethnics

i get different results to

xi:logistic i.snp sex i.ethnics

Why is that, and what is the correct way to adjust for ethnics, using i.ethnics or just ethnics?

Thank you,


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