Exploring Undercount of Young Children in 2020 Census by County – U.S. Census Bureau

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The U.S. Census Bureau used two approaches to estimate net coverage error of the 2020 Census:

  • DA, which uses birth and death records, data on international migration, and Medicare Enrollment data to produce population estimates that are compared to the census to estimate net coverage error.
  • The Post-Enumeration Survey (PES) was conducted shortly after the census data were collected. The results of the survey were matched to the census results to produce dual-system estimates of the population, which are compared to the census to produce net coverage error estimates.

While both methods have their strengths and limitations, DA is a better approach for assessing the census count of young children because the estimate comes primarily from the birth records, which are considered 100% complete in the United States.

The Census Bureau produced a series of experimental DA net coverage error estimates for children ages 0 to 4 at the state and county levels.

These estimates are experimental because they were developed with some new data and methods relative to the official national-level DA estimates released in December 2020. The distinction also takes into account that there may be more error in the county estimates than in the national estimates.

The official DA net coverage error estimates for all ages are only available at the national level because we use historical birth records as the foundation for DA population estimates. The birth records include information about where a person was born, but we do not have the data and methods to track migration over the life course.

However, we are confident we can estimate migration of young children over a relatively short period. The experimental DA estimates were produced using birth records from 2015 to 2020, so we only had to estimate migration for less than five years.

There are limitations to the data and methods used for the experimental state and county estimates, which is why we are only releasing data for 1,927 of 3,143 U.S. counties with a DA 0-4 population estimate of 1,000 or more.

One issue is that births might not be geocoded correctly and thus could be assigned to a different state or county.

For example, mothers from Virginia and Maryland may give birth in the neighboring District of Columbia. If the birth is assigned to D.C. instead of the actual state of residence, the DA population estimate will be too high and show an inflated undercount for D.C.

Geocoding can also be an issue at the county level.

For example, we found some cases in which one small rural county had a large undercount and an adjacent county had a large overcount. We assume that the county with the undercount does not have a hospital and that a number of births for that county are being assigned to the neighboring county with a hospital.

There can also be errors in the other components of population change — deaths, domestic migration and international migration — but the births are the largest portion of the overall DA estimate.

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