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
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