Dear listserv members,
I am conducting a 2 x 2 repeated measures ANCOVA in which the within factor
is time (pre,post) and the between factor is group (treatment, control). I
have a covariate of general vocabulary knowledge (PPVTRAW). The dependent
measure is RFVT, a vocabulary test.
In SPSS 11.0 for Mac OS X, I conducted the analysis with no trouble and
found a significant group x time interaction.
I am interested in breaking down this interaction - that is, looking at the
effect of time within each level of the between factor (group) as well as
looking at the effect of the between factor at each level of time. I believe
I can't do this in the windows interface (am I wrong?), so I wrote up the
syntax to test these simple effects.
The syntax I used was the following:
GLM
rfvtpre rfvtpost BY grp WITH ppvtraw
/WSFACTOR = time 2 SIMPLE
/METHOD = SSTYPE(3)
/CRITERIA = ALPHA(.05)
/EMMEANS = TABLES (grp*time) COMPARE (grp)
/EMMEANS = TABLES (grp*time) COMPARE (time)
/WSDESIGN = time
/DESIGN = ppvtraw grp .
When I get my output, I find univariate simple effects for the between
factor at each time point (pre, post), and I know how to interpret this. But
I get MULTIVARIATE simple effects for the effect of time within each level
of group.
My questions are:
1) How do I interpret the multivariate simple effect? What statistic do I
report (Wilk's or F)? Or did I set something up incorrectly in my syntax? Is
it possible to re-write the syntax such that I get univariate simple effects
for the effect of time within each level of group? If so, how?
2) I also need to report the effect sizes for each of these simple effects.
What lines of syntax should I add?
By the way, I had reported an error message earlier for this problem, but I
figured out how to fix it. So please ignore my earlier post.
Thanks very much,
-- HOP
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