Hi Rick,  thanks for useful articles, I read them. As I found I can use the unique Id that I have in the dataset as "Class" and "subject" in  'PROC MIXED". Because just this Column is unique for each child. Second, my project examined children randomly in street with considering to age (between 4-12 years old), so can I use "RANDOM"? Because, I observed the behavior of children and the characteristics of their location, and I had no repetition. Third, my database includes the different categories that I can use "children Id " for each group. Could you please guide me, how can I have them in a different category in SAS. As you can see in SAS model that showed previously, I found the formula in 2 level, but I have 4 level of data (individual, behavior, physical environment and crossing situation). I don't know if this possible or not? I really interested to do this part of my project in SAS, however, i don't have enough information. I appreciate if you guide me. Best regards  Moji   ... View more