topic Is Proc Mixed the correct procedure to determine individual change over time in a multilevel model? in Statistical Procedures

Is Proc Mixed the correct procedure to determine individual change over time in a multilevel model?

he2182 — Thu, 27 Aug 2015 11:58:34 GMT

I am trying to determine whether there is within person change in marijuana use over time in a multilevel model with repeated measures (days nested within people).

The variables I have are:

marijuana - mj use per day

id - person id

studyday - 1-7 days

time01 - study day as 0 1 with even increments in between

Will this proc mixed code answer that question?

proc mixed data = mjuse covtest noitprint;

class id studyday;

model marijuana = time01 / solution cl outp = intp outpm = intpm ;

random intercept time01/ subject = id type = un g s gcorr;

repeated /subject=id type = ar(1);

run;

Re: Is Proc Mixed the correct procedure to determine individual change over time in a multilevel model?

SteveDenham — Fri, 28 Aug 2015 11:42:44 GMT

Why not:

proc mixed data = mjuse covtest noitprint;

class id studyday;

model marijuana = studyday / solution cl outp = intp outpm = intpm ;

repeated studyday /subject=id type = ar(1);

random intercept/subject=id;

run;

I guess I don't understand the coding for time01, so I automatically revert to fitting studyday.

This all assumes that marijuana is a continuous variable, and that everything has NID (0, sigma**2) errors. If your response variable is something different, or has a large skew, you may need to consider a generalized mixed model.

Steve Denham