Date:         Fri, 17 Nov 2006 12:52:19 -0500
Reply-To:     S Elgie <qq.consulting@sympatico.ca>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:         S Elgie <qq.consulting@sympatico.ca>
Subject:      Repeated measures analysis
Content-Type: text/plain; format=flowed

My clients have carried out a careful treatment evaluation study.  One of
the groups received a battery of assessment tests at time 1, then were on a
waiting list for about 16 weeks. At time 2 at the beginning of treatment
they received the identical battery of tests and then at time 3 after 16
weeks of treatment they received the battery of tests again.

I have been asked to provide an analysis which has supporting statistical
tests which would prove that the change from time 2 to time 3 (pre- to post-
treatment) was greater than the  change from time 1 to time 2 (pre- to
post-wait list).  The usual analysis of repeated measures would not answer
this exact question and so with some doubt in my mind I formed change scores
by subtracting time 2 scores from time 1 and time 3 scores from time 2.  The
results of a repeated measures analysis on these scores were quite
significant and reasonable and I wrote them up.  I might mention that this
is a sub-analysis rather than the principal analysis.

A reviewer has criticized, suggesting that I divide the time 1 and 2 scores
by the time 2 SD and the time 2 and 3 scores by the time 3 SD prior to
forming the difference scores.  This suggestion is not sitting well with me,
and I thought to ask your advice as to other possible analyses which would
answer the question posed.

With thanks,

Susan

Susan Elgie
QQ Consulting
Toronto Canada
qq.consulting@sympatico.ca