[R-sig-ME] model specification for repeated measure

[Dot Dumuid]

Dear mixed model experts,
We have a dataset of older adults. We measured their mental health (MH) 6
months before retirement and again 12 months post retirement.
At both of these time points we also measured their physical activity (PA)
(min/day), income (INC) and general health (GH).
We would like to create a model that tells us if change in physical
activity over the retirement threshold predicts change in mental health,
and we'd like to use the model to predict how much mental health is
predicted to change when physical activity is increased from perhaps 15
minutes to 60 minutes. We'd like to use a mixed model rather than just
using change (difference) scores. And we'd like to control for things like
change in general physical health and change in income.

This is what the data look like

*ID  time  MH    PA    GH    INC*
01  pre     4      15     56     560
02  pre     5      30     30    1200
..    .....     ..       ..       ..        ...
01  post   7      40     50      50
02  post   8      45     30      0

I'm not sure how best to build the model. Something like this?

model <- lmer (MH ~ PA * time + GH + INC + (1|participant.ID) )

Thank you in advance.
Dot

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