Existing data series such as births, deaths, federal tax returns, medicare enrollment, and immigration, are used to update the decennial census counts. Census Bureau population 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 general differences.
Estimates are for the past, while projections typically are for future dates.
Estimates generally use existing symptomatic data collected from various sources, while projections must assume what demographic trends will be in the future. At the Census Bureau, the population projections use the latest available estimates as starting points. In such cases, estimates are the preferred data. Previously released estimates become superseded. Revisions to estimates are usually due to revisions to input data, or changes in methodology.