Since the 1990s, New York City has published, and public officials have quoted, an estimate that there are 1 million rent-stabilized apartments in the city, giving some 2 million tenants protections from eviction and unlimited rent increases.

The estimate comes from the city’s Department of Housing Preservation and Development, or HPD, which publishes a survey on which the number is based. Rent-stabilized apartments are vital to affordable housing and thus an important gauge of the housing market.

There’s one problem with the figure, however: It could be off by as much as 20 percent.

Data provided to ProPublica by the state’s Division of Housing and Community Renewal (DHCR) — which oversees rent-stabilized apartments — shows that, as of 2014, New York City had 839,797 rent-stabilized apartments registered with the state. For that same year, HPD’s survey estimated 1,029,918 units.