From the NY Times:
...an informal house-to-house New York Times survey of three representative square blocks where the Census Bureau said vacancies had increased and the population had declined since 2000 suggests that the city’s outrage is somewhat justified. In those blocks alone, census takers appear to have missed dozens of New Yorkers and to have overestimated the number of vacant apartments.
Often, though, owners of illegally divided houses are reluctant to disclose the number of tenants, who tend to include people who are in the country illegally and are leery of providing any information to the government. A visit from Times reporters may have proved less intimidating to landlords and residents.
City officials say as many as 80,000 residents appear to have been systematically overlooked in crowded immigrant neighborhoods like East Elmhurst and Jackson Heights in Queens and Sunset Park, Bay Ridge and Bensonhurst in Brooklyn.
One census taker who not only lives in East Elmhurst, but was also assigned to make follow-up visits to neighborhood addresses to determine if they were occupied, acknowledged the challenges of trying to find people who do not necessarily want to be found.