I am planning the analysis of a randomized, 2-arm, parallel-group treatment
study with serial measurements for the the continuous outcome variable of
interest. Let's assume that we have treatment and 2 other fixed factors, and
that there will be a (pre-randomization) baseline assessment (Visit 0) and
10 follow-up assessments at monthly intervals (Visits 1 - 10) for the
dependent variable.
Normally, I would specify a MMRM (using the Mixed procedure) that includes
the post-baseline measurements of the outcome of interest as the dependent
variable (i. e., I would include a Time variable with visits 1 through 10)
and use the baseline value of the dependent variable as a covariate. There
may, however, be randomized subjects who will drop out before the first
post-baseline assessment of the dependent variable. For regulatory reasons,
I will nevertheless have to include these patients into the model at least
in a sensitivity analysis.
My idea was now to omit the baseline value of the dependent variable as a
covariate, and to let the trajectories start at Visit 0 instead of at Visit
1. This would, however, imply that the treatment effect still needs to be
modeled starting at Visit 1, i. e., without the treatment group difference
at baseline (vVsit 0).
How do I specify this in SPSS Mixed?
Thank you very much,
Andreas
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