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Date:         Fri, 23 Sep 2005 21:59:28 -0300
Reply-To:     Hector Maletta <hmaletta@fibertel.com.ar>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:         Hector Maletta <hmaletta@fibertel.com.ar>
Subject:      Re: GLM Repeated with Categorical Covariates
Comments: To: Stephen Salbod <ssalbod@pace.edu>
In-Reply-To:  <200509240014.j8O0EkqJ007176@listserv.cc.uga.edu>
Content-Type: text/plain; charset="US-ASCII"

Covariates are supposed to be interval-level variables (they might be dummies), whereas categorical variables with several categories should be treated as factors, not as covariates. However, if you convert your ethnicity variable into a set of three dummies representing the four categories, you may use the dummies as covariates. Not strictly kosher but acceptable.

Hector

> -----Original Message----- > From: SPSSX(r) Discussion [mailto:SPSSX-L@LISTSERV.UGA.EDU] > On Behalf Of Stephen Salbod > Sent: Friday, September 23, 2005 9:15 PM > To: SPSSX-L@LISTSERV.UGA.EDU > Subject: GLM Repeated with Categorical Covariates > > A Friday question: I want to us ethnicity (4 levels) as a > covariate in an ANCOVA procedure. I think the code is > correct, but since I've never worked with a set, as > covariate, I'm a little concerned. The Syntax Guide had > little to say on this issue (see GLM: Repeated Measures, 726-734). > > > > COMPUTE Ethnic1 = 0. > > COMPUTE Ethnic2 = 0. > > COMPUTE Ethnic3 = 0. > > > > DO IF (Ethnic EQ 1). > > - COMPUTE Ethnic1 = 1. > > ELSE IF (Ethnic EQ 2). > > - COMPUTE Ethnic2 = 1. > > ELSE IF (Ethnic EQ 3). > > - COMPUTE Ethnic3 = 1. > > END IF. > > > > GLM > > pretest posttest BY group WITH ethnic1 ethnic2 ethnic3 > > /WSFACTOR = time 2 > > /METHOD = SSTYPE(3) > > /EMMEANS = TABLES(group*time) WITH(ethnic1=MEAN > ethnic2=MEAN ethnic3=MEAN) > > /CRITERIA = ALPHA(.05) > > /WSDESIGN = time > > /DESIGN = ethnic1 ethnic2 ethnic3 group . > > > > Any suggestions will be most welcome. > > > > TIA. > > > > Stephen Salbod, Pace University, NYC > > __________ Informacisn de NOD32 1.1230 (20050922) __________ > > Este mensaje ha sido analizado con NOD32 Antivirus System > http://www.nod32.com > >


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