The size of a market is based on factors influencing both demand and supply. Changes in market size, therefore, provide valuable information about the net effects of movements in both parts of the market. For example, while the number of users may decrease in response to a prevention policy targeting initiation, total expenditures may simultaneously increase due to factors shifting more light users into heavy use or an increase in supply. Therefore, estimating the size of the market, in terms of both participants and expenditures, is critical to fully understanding the impact of interventions intended to influence demand and/or supply.