In order to utilize the quantitative standards it is necessary to make a meaningful estimate of the population served by the library. The Wisconsin Division for Libraries, Technology, and Community Learning encourages "all libraries to make an estimate of of their extended service population as a point of reference for use of these standards and for other planning purposes."
The Wisconsin Public Library Standards describe four methods for estimating a library's service population. The four methods are discussed below. In addition, service populations, based on three of the methods, are reported for all OWLS member libraries.
- County Nonresident Circulation
- System Nonresident Circulation
- Local Library Nonresident Circulation
- Population of Jurisdictions Served
- Estimates for OWLS Libraries
A library's service population is based on its share of the total circulation to county residents who live outside of communities that operate libraries. For example, if a public library accounts for 30% of the total circulation (from all county libraries) to county residents without local libraries, then 30% of the county nonresident population would be assigned to that library. The library's county nonresident population is then added to its municipal population to get an estimate of its service population.
This method is similar to the county nonresident circulation method, but it is based on a library's share of the total system nonresident circulation, i.e., circulation to all residents of the system who live outside of jurisdictions that operate libraries. The library's share is applied to the systemwide nonresident population to determine its nonresident population. Once again the nonresident population is added to the municipal population to get the library's estimated service population.
This method may be more accurate for libraries located near county lines within a system because it disregards those county boundaries, e.g., New London in OWLS. However, it still does not include any residents of other systems or other communities with libraries that the library may serve.
It is possible for a library to extrapolate its service population from its own nonresident circulation patterns. The underlying assumption of this method is that nonresidents borrow materials at approximately the same rate per capita as residents. Advantages of this method are that it is not affected by any other library's activities and all political boundaries are irrelevant. It should be noted that if all libraries in a system use this method, the total service population is likely to be greater than the actual system population because residents of other systems may be included and some system residents may be counted by more than one library.
To calculate service population using this method, a library must first determine what percentage of its total circulation is to residents. The library's municipal population is then divided by that percentage, and the result is an estimate of the library's service population.
If a library believes that the majority of residents of an adjacent town (or towns) use the library, it can simply add the population of the town (or towns) to its municipal population to arrive at an estimated service population. This method is not likely to be appropriate in areas where libraries are located in close proximity because some portion of the residents of nearby towns are likely to be served by each library.
The spreadsheet attached below provides the estimated service populations for OWLS member libraries using three of the four methods described above. Data used to estimate these service populations comes from Department of Administration Final Population Estimates, January 1, 2014 and OWLS 2014 Circulation Summary [pdf].