Panel data: subjects'
value are repeatedly measured over
time points. Suppose the first half was the pretest (
grp=0) and the second half received the intervention (
Take this post as an example:
ID | time | grp | value | s_1| 1 | 0 | Q_11 | s_1| 2 | 0 | Q_12 | s_1| 3 | 0 | Q_13 | s_1| 4 | 1 | Q_14 | s_1| 5 | 1 | Q_15 | s_1| 6 | 1 | Q_16 |
Now, run a linear mixed-effecs regression
value ~ time * factor(grp) + (time | ID). Each subject
ID will have its own intercept and slope over time.
However, my concern is that
grp can be derived from
grp = ifelse(time <=3, 0, 1). This may be collinearity (I wonder if that's the exact word to use because one variable is categorical).
How do I fix the issue or even conduct another model? Thanks.