The undercount estimate has generally gone down, but it has always been disproportionately higher for blacks than for others.
There are a variety of reasons for undercounts: people move around; people may not know or be reluctant to answer government questions; address lists may be inaccurate; extremely crowded areas can be harder to count, as can extremely isolated areas. Experts believe some of those factors weigh more heavily on minority undercounts, particularly the challenges of counting in urban areas.
The 1940 census was long known to have a black undercount. Evidence of it was found within a decade in a demographic study of young children and another of draft-age men. But modern-day genealogists digging into the newly released 1940 records may be rediscovering it when they cannot locate their relatives or friends.
The absence of Gibson and her family in the available records points to an omission.
It can be difficult to find entries in the 1940 census, since a complete name index for the records won't be available for a few months longer. But Lillian Chisholm, Gibson's sole surviving sister, who was born in August 1940, confirmed the family lived at 135 W. 143rd St. at that time, making it possible to look up the census ledger.
An enumerator visited the building on at least five occasions in April 1940, according to the census records. An Associated Press review of the records found no listing of Gibson, who was 12 at the time, or her parents, at that address, though other building residents were counted.
Government officials were able to see that the count was off, particularly in the count of black men of a certain age group in the South, because they were using census data to plan for how many would be registering to fight in World War II, said Phil Sparks, former associate director of the bureau and now codirector of the Census Project, which advocates for an accurate count. More signed up than were expected.
According to census reports, the black undercount was estimated at 8.4 percent in 1940, meaning that a population counted at 12.9 million was more like 14.1 million.