Questions regarding Juvenile Arthritis Study

1. Do you want to analyze weight and height together, say BMI or separately? And the same with frequency and dose of corticosteroid, do you want to analyze them together as does per week/month, or treat as two explanatory variables? 2. I wonder whether they will also collect data from juveniles without rheumatoid arthritis for their height and weight, because I assume there might be strong effect modifiers, such as diet structure and age(juveniles usually grow faster in height and weight). So instead of taking many potential variables/confounders in the model, it may be better to control them in the data collection (case-control cohort?)