Date: Mon, 29 Mar 2010 11:06:48 -0400
Reply-To: Gene Maguin <emaguin@buffalo.edu>
Sender: "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From: Gene Maguin <emaguin@buffalo.edu>
Subject: Re: Question about contrast coding options
In-Reply-To: <917897.37766.qm@web32407.mail.mud.yahoo.com>
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Jill,
I think a better way is to build two contrast variables. The first variable
contrasts ASC+VMP=1 vs control=-1 and the second contrasts ASC=1 versus
VMP=-1 (or vice-versa), with control=0. These are orthogonal contrasts. If I
remember correctly, Cohen calls this scheme an 'effects' coding, and, as I
read the description of available contrasts, it is not available in spss
logistic. BUT, note that the computation of the odds ratios are not what
they would be with a 0-1 coding scheme because you have to take into account
the -1 coefficient.
Gene Maguin
>>> Hello everyone. I have a contrast coding question that I'd
appreciate help with to make sure I'm choosing the most appropriate option.
Briefly, I want to compare risk factors for mesh extrusion (binary
outcome) in women who've undergone one of two surgical techniques (ASC
versus VMP). For my logistic regression, I need to compare (and report odds
ratios for) the following: 1) ASC and VMP together ("GroupALL" versus
controls who didn't need mesh extrusion); and 2) the separate effects of ASC
versus VMP.
Would something like Helmert coding allow me to accomplish my goal,
since this takes into account the average effect of all subsequent
categories? Or do I need to create a "GroupALL" code for the combined effect
within a single "Group" predictor that already includes "ASC" and "VMP"?
Thanks very much for your help.
Jill
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