Posted by
Zdaniuk, Bozena-3 on
Mar 16, 2021; 6:56pm
URL: http://spssx-discussion.165.s1.nabble.com/Interpretation-of-interaction-with-time-varying-predictor-Growth-model-tp5740294p5740313.html
Hi Oliver,
When you use time as categorical var, you should get the estimates of the interaction between depression and each time point compared to the reference time point. So, if you have four points, you should get three estimates of interaction. Those estimates tell you the nature and significance of the interaction (which time point interacts significantly with depression comparing to the ref time point). So if the estimate of T2 x Depr is significant you know that the difference between t2 and t1 depends on the values of depression (assuming t1 is your ref point) and that coefficient tells you about the direction of that interaction and its magnitude - all you need to know to interpret the interaction.
Cheers,
Bozena
-----Original Message-----
From: SPSSX(r) Discussion <
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Sent: March 16, 2021 11:28 AM
To:
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Subject: Re: Interpretation of interaction with time-varying predictor (Growth model)
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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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