I normally use
xtmixed in Stata to test hierarchical linear models (e.g. performance of students nested in schools).
Now it's the first time I need to test a longitudinal model where I have 3 waves of repeated measures. My measured variables refer to students' characteristics (i.e. sex, performance in each wave, hours of study in each wave, etc..) . I'd like to build a model where the first level is repeated time measures and the second level is each student. I have two problems:
- Not every students participated to all the three waves;
- The three waves are not equally spaced in time (i.e second one is 1 year after the first one and the third one is 6 months after the second one).
I would need your help to know if:
- having not equally spaced waves is a problem? Do I need to specify it in some way in Stata?
how do I handle missing data? Can I just put 2 observations for one student (who missed the second wave) and 3 for another (who did not miss any wave)?
Can I build a model like this?
xtmixed performance sex studyhours || StudentID:, var mlOr do I need to include a variable taking into account the time?
Actually, I don't really care about the influence of time. My research question is about sex and study hours influencing performance. But I saw many cases where researchers include interactions with a variable specifying time.