Posted by
Oliver on
Mar 20, 2021; 7:19pm
URL: http://spssx-discussion.165.s1.nabble.com/Interpretation-of-interaction-with-time-varying-predictor-Growth-model-tp5740294p5740341.html
Bruce and Bozena,
Thanks to both of you for your great input. I've followed Bozena's
suggestion (i.e., to re-code the order to Waves as a way to have the right
reference category) and this works well. Thanks Bozena !
I've also followed Bruce's syntax suggestion to interpret the Level 1 (Time
* Dep) interaction effect on pain, and this works great as well. As
suggested, the effect of the Time (IV; categorical) on pain (outcome; 0-10)
was tested at different levels of the moderator (dep). Consistent with Aiken
& West, I chosen values corresponding to -1SD, Mean, and +1SD.
MIXED Pain BY Time WITH Dep
/METHOD = REML
/PRINT = SOLUTION TESTCOV
/FIXED = Time Dep Time * Dep | SSTYP(3)
/RANDOM = INTERCEPT | SUBJECT(ID) COVTYPE(UN)
/REPEATED = Wave | SUBJECT(ID) COVTYPE(AR1)
/EMMEANS TABLES(Time) WITH(Dep=0.15) COMPARE(Time) ADJ(SIDAK)
/EMMEANS TABLES(Time) WITH(Dep=2.85) COMPARE(Time) ADJ(SIDAK)
/EMMEANS TABLES(Time) WITH(Dep=5.55) COMPARE(Time) ADJ(SIDAK)
The syntax (and output) proposed by Bruce is extremely useful for the
interpretation of the Level 1 interaction effect. My question at this point
is this: The moderator values (-1SD, M, +1SD) were computed based on the
"Grand mean". For a specific question of interest in my study, I'd like to
know if the time (IV) effect on pain (outcome) varied as a function of
within-person changes in the moderator (dep) at each of the time points.
Would it be possible to change the specification (/EMMEANS line) with
"cluster/within-person" centered scores ? For instance, perhaps the degree
of within-person changes in pain across time points varies as a function of
concurrent (within-person) changes in the moderator. This If so, would
someone know how to perform this ?
Thanks again everyone,
O.
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