Existing data series such as births, deaths, Federal tax returns, medicare enrollment, and immigration, are used to update the decennial census base counts. PEP estimates are used in Federal funding allocations, in setting the levels of national surveys, and in monitoring recent demographic changes. A methodology reference accompanies most of our population estimates offerings. There is not a distinct dichotomy between population estimates and population projections, but there are some differences in time reference and derivation.
Estimates usually are for the past, while projections typically are for future dates.
Projections must assume future trends for fertility, mortality, and other demographic processes. At the Census Bureau, the population projections use the latest available estimates as starting points. In our current product offerings the user may see both an estimate and a projection available for the same reference date, which may not agree because they were produced at different times. In such cases, estimates are the preferred data. Revisions to estimates and geographic detail With each new issue of July 1 estimates, PEP revises estimates for years back to the last census.