```Date: Fri, 20 Aug 2004 17:01:31 -0400 Reply-To: Richard Ristow Sender: "SPSSX(r) Discussion" From: Richard Ristow Subject: Re: Repeated measures control for baseline scores Comments: To: Marie-Claire In-Reply-To: <000a01c4844c\$c9b24160\$6aa1d781@ed.ac.uk> Content-Type: text/plain; charset="us-ascii"; format=flowed I'm hesitant, being far from the best mathematical statistician on the list (Hi, Marta!), but nobody else has responded -- At 07:24 AM 8/17/2004, Marie-Claire wrote: >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. On the face of it, this makes perfect sense. >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? I'm assuming your "time 1" is the baseline. If it is not, address the question by adding it to the analysis as a third level of the within-subject variable. But if "time 1" is the baseline, I very much believe you're right: the within-subject effect has two degrees of freedom, of which a useful representation is the mean of the two values, and their difference. ```

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