Date:         Tue, 17 Aug 2004 12:24:40 +0100
Reply-To:     Marie-Claire <M.Whyte@sms.ed.ac.uk>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:         Marie-Claire <M.Whyte@sms.ed.ac.uk>
Subject:      Repeated measures control for baseline scores
Content-Type: text/plain; charset="iso-8859-1"

I am wanting to compare 3 experimental groups with a control group on measures of cognitive performance at 2 time points.  

I have previously analysed this data using a repeated measures ANCOVA in SPSS (covarying for differing times between the 2 assessment points between groups), with group as a between-subject variable and cognitive perfromance at time 1 and at time 2 as the two levels of the within-subject variable.

I have been advised recently however to amend this analysis by controlling for differences between the groups at baseline, such that all groups are equated on cognitive performance at time 1 and from here any changes to time 2 can be seen across all groups (main effect of 'time') or differentially in one particular group relative to all others (group*time).

My question is, are baseline scores not already held constant in the repeated measures ANCOVA model, or do I need to redo all analyses using a linear regression model in SPSS instead?

Regards

Marie-Claire Whyte
Department of Psychiatry
Kennedy Tower
Royal Edinburgh Hospital
Morningside
Edinburgh
EH10 5HF
0131 537 6258
M.Whyte@sms.ed.ac.uk