I’m doing my thesis analyses with a longitudinal data set in which the time between visit dates varies by individual and not everyone has the same number of visit dates. I am interested in assessing whether metabolic syndrome (binary variable) impacts mental health-related quality of life (continuous) and severity of depressive symptoms (binary/continuous). I know using a mixed model is the best option for both of these aims, but how do I determine which is the best model to use? There are also missing values for each of the variables of interest. I would appreciate any feedback.