I am just starting out with PROC MIXED and unable to find help at my university, so hoping someone here can help!
I am setting up a model where my outcome (weight) is measured 3 times (baseline, 12m, 18m) there is so much variability with the time weight was actually measured (baseline-68months) that I am using a continuous "time since intervention" variable.
1. Do I need to center my time variable? why?
2. Is there a general rule of thumb on when to use random slope v random intercept v both random slope and intercept? When I run it with random intercept + slope the model shows no interaction between gender*time, while the random intercept only model does—any thoughts on why this might be?
My base model is:
Weight= time since intervention
and my model w covariates
weight= time since intervention +gender + age
I was going to include a time*time var since we believe people lose more weight earlier in the study- does this make sense?
When I DON’T center time this time*time var is significant vs not signif. when I do center time- any idea what this means?
i want to test time*gender to see if the rate of weight loss differs by gender and will add that in the next model. - does this make sense? does the time variable in this case need to be time*time*gender?
Any help or thoughts in general re mixed models is appreciated.
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