One of the most important activities for completing a budget-impact analysis is understanding the population dynamics in order to estimate the size of the population eligible for the new drug. Determining those who are eligible for the new drug in the jurisdiction(s) of interest is a key determinant of the changes in costs and outcomes that may occur from the budget holder’s perspective. The credibility of the analysis will depend on the model correctly identifying those eligible for the new drug based on the treatment pathway in each jurisdiction. In this chapter, we present methods for estimating the size of the eligible incident and prevalent populations, including changes in the size and condition severity mix of these populations over the analysis time horizon. Issues around patient subgroups and catch-up are also presented.
