About the Estimates Base

Revisions to estimates and geographic detail

With each new issue of July 1 estimates, the population estimates program revises estimates for years back to the last census. Previously released estimates are superseded. Revisions to estimates are usually due to revised or updated administrative input data, changes in methodology, or legal boundary changes, especially for subcounty units of government, such as incorporated places. The frequency of estimates and availability of demographic detail vary by geographic level.

Geographic Boundary Updates

In many cases a legal entity has had substantial changes to its boundaries between 1990 and Census 2000. In these cases, a comparison of the 1990 census result with the Census 2000 count shows population change due not only to demographic events (births, deaths and migration) but due also to geographic events (annexations, deannexations, etc.).

In contrast, all boundary updates in the post-Census 2000 population estimates are reflected in the 2000 population estimates base and not the year in which the change occurred. Therefore the post-census population estimates show population change caused by demographic events only. A boundary change will make the estimates base for an area larger or smaller, but it will not cause a population estimate to show population growth over time. Population Division produces tables that include both the Census 2000 count and the 2000 population estimates base and indicate the amount of population change due to boundary changes.