We include all residents in cities, counties, and the state in the population estimates. Residents include people living in their own homes, rental homes or apartments, mobile homes, or in college dormitories, nursing homes, jails and prisons, and other group quarters. We consider college students to be residents of the place where they live while attending college. We count seasonal residents as living at the place that they consider to be their usual residence.
For estimates for the state population, we rely on the registration of births and deaths for an estimate of natural increase (the excess of births minus deaths). We use school enrollments data for an estimate of net migration for the population under 18 years, non-farm payroll information for the population aged 18 to 64, and Medicare data for the population aged 65 and older.
We base county population estimates on a method called ratio correlation. The ratio correlation method relates changes since the last census to changes in state tax returns, school enrollments, births, and voter registration. We track changes in each of these types of administrative data to make an estimate of the current county populations. We also examine changes in the housing stock as a check on the accuracy of the county estimates.
We determine city population estimates based on changes in the housing stock. City officials report each year on the changes in their city's housing stock (for single family units, multi-unit residents, and mobile homes) and the number of persons residing in group quarters. We also take into account any population changes that have been due to annexations. We separately estimate possible changes due to the number of people per housing unit and use this information to prepare the city's final population estimate.
We monitor city annexations throughout the year. We are informed about annexations by the office of the Secretary of State. When an annexation occurs, we contact city officials to find out the number of residential housing units and population, if any, involved in the annexation. If the annexation added people to the city's population, then we certify an updated city population on a quarterly basis, in a special report to the Secretary of State's office.
As required by Oregon administrative rules, all city annexations that result in a change of population must be certified by the Population Research Center. In practice, the certification for populations less than 125 housing units involved a count of housing units and people that is conducted by city officials and reviewed by Population Research Center staff. If the annexation is large and involves more than 125 housing units, city officials should contact the Population Research Center as soon as possible. Population Research Center welcomes the opportunity to work with city officials so that the annexation certification can be accomplished promptly. Please visit our page on Enumerations for additional information on this topic.
