Market definition can be achieved via a Sales, Epi, Opportunity or Patient Flow based forecast
Sales based forecasts are effective for short-term tactical forecasting for "stable markets".
Epi Patient Based
Epidemiology based forecasts start with a population and by applying a series of filters estimates the size of target patients.
Splits treated patients into stable vs opportunity groups (e.g. new, switch).
Patient Flow Based
Follows a cohort of patients over time and captures progression to different disease states.