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Developing the Population Estimates

All residents of Oregon’s cities, counties, and unincorporated areas are included in the population estimates tables in this report.  Residents include people living in housing units (single-family and multi-family units; mobile homes; special housing units, such as boats, tents, motel rooms), and in group quarter facilities (such as college dormitories, nursing homes, jails and prisons).  We count residents as persons living at the place that they consider to be their usual residence, or where they are residing at least 6 months of the year.

Toward the goal to develop accurate population estimates, we work closely with state, county, and city officials. We request annual information from them and invite their review of our methods and estimates.

To estimate the State population, we use the following equation that defines population in the current year as the population in the most recent U.S. Census plus the sum of two basic components, natural increase and net migration, that occurred since Census.

Pop.(current year)

           Pop. (2010)   +  Nat. Incr.(post-2010)   +  Net Mig.(post-2010)

We rely on the registration of births and deaths to estimate Oregon’s natural increase (the number of births minus deaths). To estimate the net migration (in-migrants minus out-migrants), we use data on school enrollment, employment, labor force, state and federal income tax exemptions, driver license issuances, voter registration, and Medicare enrollees. We use a regression method that relates the changes in these data since the last Census to changes in population and net migration during the same time period.

We use a composite of two methods to estimate county population: 1) a ratio correlation method, and 2) a modified component method.  In the ratio correlation method, we track changes in state and federal income tax exemptions, school enrollment, the number of births, driver license issuances, and Medicare enrollment data to produce an estimate of the current county populations. In the component method, we determine the natural increase for each county, and estimate net migration based on recent historical trends. In addition, we examine changes in the housing stock to verify the accuracy of the county estimates and make adjustments, if necessary.

We develop population estimates for Oregon’s cities according to the changes in their housing stock and group quarters population.  City officials report to PRC each year the changes in their city's housing stock (for single-family units, multi-family units, and mobile homes) and the number of persons residing in group quarters facilities.  For each city, we assume an average number of persons per housing unit (PPHU) by housing type calculated from the most recent census data collected. Applying the PPHU to the current housing stock and adding group quarters population, we arrive the city-level population estimate.  We take into account any population changes due to annexations to produce the city's final population estimate. Please note that if a city has not submitted recent data, we hold its population constant to its previously certified estimate.

The number of persons residing in the county unincorporated areas is estimated by calculating the residual of the total county population and the sum of the cities’ populations in the same county.  Some adjustments may be made according to recent data on housing growth and changes in group quarter population that county officials provide us. Use caution when comparing the population estimates of the unincorporated county areas over time, and note that they represent revised estimates rather than estimates resulting from direct change during the year. 

We monitor city annexations throughout the year.  We are notified about annexations by the office of the Secretary of State.  When an annexation occurs, we contact city officials to request the number of residential housing units and population, if any, that were involved in the annexation.  If the annexation added to the city's population, we certify an updated city population, which is done on a quarterly basis. Each quarter we publish online the revised populations in a supplement to our annual population estimates report.

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 involve 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.

In mid-November each year, we make available to the public the preliminary annual July 1 population estimates for Oregon and its cities and counties. There is a one-month review period during which we receive feedback from state, city and county officials and other community partners. During this time we may make additional adjustments to the estimates prior to certification. We certify the final July 1 population estimates December 15th.  We publish the state, city and county estimates on our website shortly after.  This annual population report, which includes the complete set of population estimates, is usually available online in the spring of the following year.