Hello.
I am running a multivariate multilevel mixed effects model, and am trying
to understand what the interaction term tells me.
A very simplified version of the model looks like this:
model <- lmer (phq ~ -1 + as.factor(index_phq) * Neuro + ( -1 +
as.factor(index_phq)|UserID), data=data)
The phq variable is a categorical depression score on 9 depression items
(classified by the variable "index_phq"), and Neuroticism is a covariate
(random effect = the subjects of my study, they should be allowed to vary
in slope/intercept).
The output looks like this:
Fixed effects:
Estimate Std.Error t-value
as.factor(index_phq)1 0.0185335 0.0466784 0.397
as.factor(index_phq)2 -0.0327630 0.0453493 -0.722
as.factor(index_phq)3 0.2210580 0.0612363 3.610
as.factor(index_phq)4 0.6240455 0.0582654 10.710
as.factor(index_phq)5 0.1121727 0.0622298 1.803
as.factor(index_phq)6 -0.1797495 0.0484940 -3.707
as.factor(index_phq)7 -0.0907630 0.0488517 -1.858
as.factor(index_phq)8 -0.0854370 0.0384632 -2.221
as.factor(index_phq)9 -0.0621792 0.0319729 -1.945
Neuro 0.0218704 0.0019690 11.107
as.factor(index_phq)2:Neuro 0.0026006 0.0018268 1.424
as.factor(index_phq)3:Neuro 0.0012185 0.0025774 0.473
as.factor(index_phq)4:Neuro 0.0009458 0.0023336 0.405
as.factor(index_phq)5:Neuro 0.0061280 0.0025547 2.399
as.factor(index_phq)6:Neuro 0.0070453 0.0018788 3.750
as.factor(index_phq)7:Neuro -0.0008890 0.0020755 -0.428
as.factor(index_phq)8:Neuro -0.0092411 0.0020197 -4.575
as.factor(index_phq)9:Neuro -0.0159498 0.0021470 -7.429
Looking only at the interaction effects, do I understand correctly that ...
1) Neuroticism has a significant effect on all PHQ items, because the
effect is significant on phq1 (t-value=11.107), and the other t-values are
at a maximum -7.429 lower than the t-value of 11.107, meaning they are
still significant because they are still all above t~2?
2) The strongest effect is on PHQ6, which is significantly higher than the
effect from Neuro on PHQ1?
3) The weakest effect is on PHQ9, which is significantly lower than the
effect from Neuro on PHQ1?
Thank you
Eiko
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