Posted by
Oliver on
Mar 16, 2021; 6:28pm
URL: http://spssx-discussion.165.s1.nabble.com/Interpretation-of-interaction-with-time-varying-predictor-Growth-model-tp5740294p5740312.html
Rich & Bruce,
Thanks for your very useful input. This is much appreciated !
As suggested, I've coded "Time" as a categorical variable using "BY", and
results revealed a significant main effect of Time (p = .000). I've then
used the /TEST subcommand to perform planned contrasts to help data
interpretation of the "Time" effect on pain. Assuming that I did it
correclty, I conducted the Helmert contrast, as suggested by Bruce (i.e.,
comparing pain at each time point versus the mean of all other subsequent
time points.
MIXED Pain BY Time
/METHOD = REML
/PRINT = SOLUTION TESTCOV
/FIXED =
Time
| SSTYP(3)
/RANDOM = INTERCEPT | SUBJECT(ID) COVTYPE(UN)
/REPEATED = Wave | SUBJECT(ID) COVTYPE(AR1)
/EMMEANS = TABLES(Time)
/TEST = "Planned contrasts of TimeBaseline versus subsequent" time 1 -1/4
-1/4 -1/4 -1/4
/TEST = "Planned contrasts of 3m versus subsequent" time 0 1 -1/3 -1/3 -1/3
/TEST = "Planned contrasts of 6m versus subsequent" time 0 0 1 -1/2 -1/2
/TEST = "Planned contrasts of 9m versus subsequent" time 0 0 0 1 -1
*At this point what I'm wondering is how to interpret the Time * Depression
(Dep) interaction effect. When coding "Time" as categorical, the
interaction effect (i.e., Time * Dep) is significant. But I don't know how
to use planned contrasts to probe the interaction effect. For instance, it
is possible that depression scores (continuous: 0-10) influenced pain
(continous: 0-10) but only between the baseline and the 3 month follow-up
time point. I read on the UCLA website that "*interaction contrasts*" could
be used
(
https://stats.idre.ucla.edu/spss/faq/how-can-i-test-contrasts-and-interaction-contrasts-in-a-mixed-model/),
but for some reason I can't figure out how to apply the coding to my Time *
Dep interaction.
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)
By any chance, does someone here know how to perform the "interaction
contrasts" ? Any assistance would be tremendously appreciated, as usual.
O.
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