Chapter 3 Overview of the Cohort-Component Method We begin our discussion of population projection methods with the cohort-component method, which has a longstanding tradition in demography (Bowley 1924; Cannan 1895; Whelpton 1928). The Census Bureau began using this method for national projections in the 1940s and for state projections in the 1950s and has used some version of the method ever since. A survey conducted by the Federal-State Cooper- ative Program for Population Projections found that 89% of states making state-level projections of total population used some form of the cohort-component method; for states making projections by age, sex, and race, 95% used the cohort-component method (Judson 1997). It is also widely used for projections at the county and subcounty level. Although current applications of the cohort-component method are more detailed and sophisticated than the earliest applications, its basic framework is much like it was 100 years ago. The cohort-component method is so widely used because it provides a flexible yet powerful approach to population projection. It can incorporate many application techniques, types of data, and assumptions regarding future population change. It can be used at any level of geography, from nations down to states, counties, and subcounty areas. Perhaps most important, it provides projections not only of total population but also of demographic composition and individual components of growth. The cohort-component method provides a good starting point for the study of state and local population projections. 3.1 Concepts and Terminology A cohort may be defined as a group of people who experience the same demographic event during a particular period of time and who may be identified at later dates on the basis of this common experience (Shryock and Siegel 1973, p. 712). For example, all babies born during the 1990s comprise the birth cohort for that decade; all persons married in 2005 form the marriage cohort for that year; and all immigrants entering the United States between 2010 and 2012 make up the immigration cohort for that S.K. Smith et al., A Practitioner’s Guide to State and Local Population Projections, The Springer Series on Demographic Methods and Population Analysis 37, DOI 10.1007/978-94-007-7551-0_3, © Springer Science+Business Media Dordrecht 2013 45