WISH: Annual Population Estimates for Wisconsin Counties - Methods
Estimates from 2020 and after
The population estimates for 2020 comes from the Vintage 2025 population estimates provided by the Population Division at the United States Census Bureau. The Population Estimates Program sent a detailed version of their Vintage 2025 series to the Department of Health Services for the purpose of generating health statistics and determining funding allocation.
The population estimates are based on a July 1 reference date as opposed to the April 1 reference date used for the decennial census, which explains some differences from the decennial census counts. Additionally, the population estimates are adjusted based on age heaping, child undercount, challenges to the census counts, and the re-assignment of the “Some Other Race” category to other races. More information can be found in the Vintage 2025 population estimates methodology statement (PDF).
Estimates from 2010 to 2019
The estimates from 2010 to 2019 are similarly produced by the Population Estimates Program within the Population Division of the United States Census Bureau. These estimates are gathered from a detailed version of the 2010-2020 Intercensal Estimates that is provided to the Department of Health Services.
Estimates from 1990 to 2009
The Office of Health Informatics (OHI) produced population estimates by sex and age group for Wisconsin counties and the state in non-Census years. The reference date for each set of annual estimates was July 1.
The estimates were produced through iterative proportional fitting, or "raking". The procedure adjusted a matrix of cells in an iterative fashion until the matrix converged with a separate set of marginal totals. The process continued until the internal cells converged with the totals for all dimensions (in this case, age group and sex) simultaneously. The term "raking" roughly describes how the matrix cells were distributed along each dimension until the matrix fit the marginal totals.
The data used to produce the estimates came from the U.S. Census Bureau and the Wisconsin Department of Administration. The Census Bureau's state age-by-sex estimates were used along with county total estimates and county estimates by sex by five-year age group projections from the Wisconsin Department of Administration's Demographic Services Center.