Hi,
I have a few questions about longitudinal analysis with different start time and time intervals.
In my study, I have six time points where BMI is measured in two groups of participants:
Time1: Beginning of school year 1.
Time2: End of school year 1.
Time3: Beginning of school year 2.
Time4: End of school year 2.
Time5: Beginning of school year 3.
Time6: End of school year 3.
There is summer break between each end of school year and beginning of next school year (eg. Summer break between time 2 and time 3). Currently, I used mixed model and only included participants who completed all six timepoint:
proc mixed data=data method=ML;
class id time(ref="1") group(ref="1") ;
model bmi = time group time*group/ s outp=predicted residual;
repeated time/type=un subject=id ;
run;
As the study continues with more time add down the road, I want to expand my sample by including 1) participants with different start point, such as including people who only start at time 3; and 2) people who miss one school year in between, such as people who participate in school year 1 and 3, but miss school year 2.
Do you have any tips on how to modify the model?
Thanks!
SY
