The population of Puerto Rico was overcounted by 5.7%, or 174,000 people, when the Census Bureau conducted its 2020 count.
The overcount came to light Tuesday after the bureau announced it concluded a post-enumeration survey used to measure the accuracy of the census by independently surveying a sample of the population on the island. The effort is being used to inform improvements on the 2030 census.
On April 2021, the Census Bureau announced Puerto Rico’s population fell 11.8% to nearly 3.3 million over the past decade. But the latest post-enumeration survey suggests that Puerto Rico's total population may be closer to 3.1 million.
Issues with duplicate records seem to have contributed to the overcount, according to the bureau.
The estimated rate of erroneous enumerations in the 2020 census was 9.8% or 319,000. Almost all of them, about 9.1% or 294,000, could be attributed to duplicate records. In contrast, the estimated rate of erroneous enumerations due to duplication in the U.S. mainland is 1.6%.
"So you could have people counted at multiple places, and we were not able to detect that they were duplicated," Timothy Kennel, an assistant division chief for statistical methods at the Census Bureau, told NBC News.
Kennel said a "classic example" of duplication could be a college student living at a dorm who filled the census form at the university but their parents might have also put them down as part of their household in the form.
But in Puerto Rico, almost 10% of the population were counted twice, Mario Marazzi, the former executive director of the Puerto Rico Institute of Statistics and a member of the Census Scientific Advisory Committee, said in an interview with NBC News, speaking from Puerto Rico.
In a predominantly Spanish-speaking jurisdiction, people often have two last names but “may not be reporting their names consistently in every form,” since there’s not always space for it, Marazzi said. Also, date of birth numbers are written differently — in Spanish, date of birth digits start with the day, not the month — and unduplication efforts don’t always account for these inconsistencies, which may result in future overcounts.
Puerto Rico population overcounts were most prevalent among women age 50 and over (11%) and men age 50 and over (8.3%), followed by women ages 30 to 49 (4.9%).
Overall, "adult males (5.7%) and females (7.3%) were statistically significantly overcounted in the 2020 Census," according to the bureau.
Unduplication efforts in the 50 states and Washington, D.C. often use administrative records from the IRS, Social Security Administration, the U.S. Postal Service and the Medicare enrollment database to “match people in different households,” Kennel said. “That effort ended up removing some duplicates” on the U.S. mainland.
“We didn’t have the same quality of administrative records in Puerto Rico to conduct that unduplication effort,” Kennel added.
The 2010 census also overcounted Puerto Rico's population by 4.5%, or 160,000 people. At that time, the Puerto Rican population was about 3.7 million. On that census, the bureau also reported a high rate of erroneous enumerations.
The estimated rate of erroneous enumerations in the 2010 census was 7.9% or 290,000. Almost all of them, about 7.2% or 264,000, could also be attributed to duplicate records.
"We want to partner with stakeholders to think about how can we improve the census in Puerto Rico, so we do not have an overcount, nor an undercount," Kennel said. "Maybe if we had better administrative records in Puerto Rico, we could do a better job of removing some of those duplicates, detecting them and making sure we are trying to lower duplication."
Kennel also said there may be space for additional "creative ideas" that could be researched over the next decade ahead of the 2030 census.
"The post-enumeration survey seems to suggest that there will be room for improvement in Puerto Rico, especially with trying to figure out ways to reduce the overcount," he added.