I have measures of weight for three countries for 3 time points. I want to find whether there is a difference in change of weight over time, based on gender accounting for groups (country). I also have a bit different age ranges for countries, so I was thinking to use an age variable only, which would account for differences in groups. The final model would look something like this:
weight ~ gender + (age_at_weight_measure | participant_id)
Do I think in the right direction or should I anyway add a country group variable somewhere?
If I want to find the influence of variable X on the weight change (let's say, income (categorical)), would the model change into this:
weight ~ gender*income + (age_at_weight_measure | participant_id)
Data look somewhere like this: