"Correctional facilities are mission driven. Data is relatively low priority, yet it is often data, and what it reveals, that forms the basis of correctional needs assessments. Projected Daily Population (ADP), in particular, is the foundation of every needs assessment" (p. 39). This article concisely explains how to use ADP to forecast population for your agency or facility. Sections of this article cover: straight line forecasting of ADP; the inevitability of change in admissions (ADM) and average length of stay (ALOS); Little's Law—ADP=(ADM/T) x ALOS (T being the days of duration being looked at like a week, month, year); historical analysis; historical ADP, ADM, and ALOS; forecasting future ADP—admissions first; estimating future ADP; and the assumption of change.