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The Population Research Center includes 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 group quarters facilities such as college dormitories, nursing homes, and jails and prisons. We consider college students to be residents of the place where they live while attending college.  Seasonal residents are counted as living at the place 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). In addition, a combination of data on school enrollments, license issuances, employment, tax exemptions, and new housing construction is used to estimate population for the state and counties.

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. Changes in the housing stock are also examined  as a check on the accuracy of the county estimates.

City population estimates are determined  based on changes in the housing stock and on occupancy rates and the average number of persons per household. 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 occur due to annexations. Possible changes due to the number of people per housing unit are estimated separately; the resulting information is then used to prepare the city's final population estimate.

PRC monitors city annexations throughout the year. We are officially notified of annexations by the office of the Oregon Secretary of State. When an annexation has been recorded, we contact city officials to determine the number of residential housing units and population, if any, involved in the annexation. If the annexation added people to the city's population, we then certify an updated city population on a quarterly basis, in a special report to the Secretary of State's office.  The resulting revised population estimate is also available online, in quarterly supplements.

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 with less than 125 housing units involved a count of housing units and people, 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.