Projecting Population
The population projections on the CARA website are provided by NPA Data Services, Inc. (www.npadata.com), a company that develops and maintains databases of historical and projected U.S. population, employment, earnings, and personal income. The projections on this website were developed by NPA Data Services in 1999. More recent projections can be purchased from them.
In deriving county-level population projections of the type shown on this website, NPA Data Services begins with national-level projections for population, employment by industry, earnings by industry, and personal income. These population projections are based on various assumptions about birth and death rates and rates of immigration. The employment, earnings and income projections are based on economic growth models.
NPA Data Services then projects employment at the industry level for each geographic area (county, state, MSA) based on historical differences between that area's employment growth rates at the industry level and the corresponding national-level projected growth rates. Growth rate differences are assumed to narrow over time: each area's employment gradually converges to the national trend in each industry. Area projections are constrained so that they add up to the national projections.
Employment projections, along with area-specific employment/population ratio projections, are used to derive population projections for each area, on the assumption that people move to wherever the jobs are. Area projections are constrained so that they add up to the national projections.