1. A population estimate is the calculation of a current or past population, typically based on symptomatic indicators of population change, while a population projection is the numerical outcome of a particular set of assumptions regarding future population trends (Swanson and Stephan, 2004). It is worthwhile to add here that there is a distinction usually made between the term ‘projection’ and the term ‘forecast,’ with the latter being defined as a population projection deemed most accurate for the purpose of predicting future population (Swanson and Stephan, 2004). It is also useful to note here that there are two distinct traditions in regard to population estimates: (1) demographic and (2) statistical – that is, the methods used by those who do sample surveys. Demographic methods are used to develop estimates of a total population as well as the ascribed characteristics – age, race, and sex – of a given population. Statistical methods typically employ sample surveys in order to estimate the achieved characteristics of a population – educational attainment, employment status, income, and martial status, for example. Among survey statisticians, the demographer's definition of an estimate is generally termed an ‘indirect estimate’ because unlike a sample survey, the data used to construct a demographic estimate are symptomatic indicators of population change (e.g., K-12 enrollment data, births, deaths) and do not directly represent the phenomenon of interest. Among demographers, however, the term ‘indirect estimate’ has a different meaning (Swanson and Stephan, 2004). Most national statistical agencies produce estimates using both traditions, demographic and statistical. With regard to population projections, the distinction between the demographic tradition and the statistical tradition is less pronounced, however.
2. The information about the PAA's Committee on Applied Demography is largely drawn from Voss (2005).
3. A technique for making extensive population estimates;Bogue;Journal of the American Statistical Association,1950
4. Beyond Six Billion: Forecasting the World's Population,2000
5. Population estimates;Bryan,2004