Hey, One newbie asks for help building a right model for my longitudinal data analysis. I cant find help in my faculty, so you're my last hope It's my first time using SAS and building multilevel model, so my understanding is very limited. So here is my situation: 1. There are 3 years longitudinal data, kids were tested every year same time for 3 years (0,12,24). 2. There are 4 groups: kids were grouped according to their BMI: underweight -1 (20 boys), normal weight -2 (50 boys), overweight-3 (20 boys), obese-4 (23 boys). They stayed in the same BMI group for all 3 years of study. 3. My data is already converted from wide to long. 4. My predictors (fixed) should be: biological_age, biological_age2, height, weight, lean_mass, fat_mass, physical activity. 5. Biological age, and biological age2 should be added both as fixed and random effect. 6. Do you think I should center the study start to 12 month? 7. My purpose is build group specific multilevel model for 3 skeletal sites: BMD_T, BMD_FN, BMD_LS. 8. I want to analyze the relationships between PA(physical activity) and bone parameters (BMD_T, BMD_FN, BMD_LS) in boys from all 4 groups (different body composition). 9. Basically I want group-specific model for BMD_T, BMD_FN, BMD_LS controlling for biological age, height, weight and testing for the effect of PA(physical activity). I have some more predictors in the list and some more ideas, but first I would like to learn the basics, according to my situation. I am planning to use stepwise procedure. So, please if someone will find some time for me to write the model and then some time to answer my few questions about that, I would appreciate it a lot! I think if I understand the way to build model and what means all these commands, I will be able to go on myself and then help others Thank you!
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