Before discussing any public library's services, developing a meaningful and accurate estimate of the population it serves is crucial. In Wisconsin, estimating library service populations is complicated by the fact that libraries provide service to many people who do not reside within the municipality that established the library. These "nonresident borrowers" include county residents who have access to the library, residents of other municipalities within the same regional system, and in many cases, residents of other systems. A library’s true service population is almost always greater than its "official" municipal/resident population.
The Wisconsin Public Library Standards describe several methods for estimating libraries’ service populations (see Chapter 3: How to Use These Standards). The method used at the Department of Public Instruction (DPI) for calculating resident population, additional county population (formerly "additional service population"), and extended county population (formerly "extended service population") is the first method described by the Standards; i.e., Use the Service Data Population.
- Calculate libraries' total resident population from the Wisconsin Department of Administration (DOA) final estimated populations for the year.
- For a joint library—formed by more than one municipality—this is the sum of its municipal populations
- For a joint city-county library—serves one municipality and its home county—this is the county population, less the municipal population of other public libraries in the county.
- For a consolidated county library, this is the county population, less the municipal population of other public libraries in the county.
- Tribal libraries' resident population is based on the most recent decennial census.
- Use each county's total population from DOA data to calculate a nonresident county population that does not reside in library municipalities; i.e., total county population minus the total resident population.
- A library's resident population is counted toward its home county total population, regardless of whether the municipality crosses county lines or the library is formed by municipalities in more than one county.
- From annual report data for the previous year, calculate each county’s total library circulation to nonresidents living in the county who do not have a public library.
- Using each library’s total library circulation to nonresidents living in the county who do not have a public library, calculate the library’s percentage of library circulation to nonresidents living in the county who do not have a public library.
- For each library, the percentage above is applied to the county’s total nonresident population that does not reside in library municipalities. This is the library’s Additional County Population.
- Each library’s Extended County Population is its Resident Population plus Additional County Population.
Using the population estimates produced by the DOA Demographic Services Center every fall, the Department of Public Instruction's Public Library Development Team compiles an Excel file of resident populations for Wisconsin public libraries. Libraries can use this data to calculate their resident support per capita and compare it to the previous year in advance of the next annual report cycle.
Jamie McCanless, Public Library Development Team