Twenty percent of the apartments in the luxury high-rise tower going up at 626 Flatbush Ave. will be deemed affordable. But the measure used to decide what is “affordable” is often questioned.

It is based on a percentage of the federal statistic called the Area Median Income, or AMI. For the New York metropolitan area, the AMI is $84,000 for a family of four. Based on a percentage of that, rentals would be designed for families earning $33,400 to $41,750 a year.

Critics say the Area Median Income calculation should consider each community district on a case-by-case basis rather than producing an average based on the incomes of the entire region, including affluent suburbs. The median income for a family of four in Community District 9 is about $41,000, according to census data, about half the value for the metropolitan area.

Edward Snajdr, a professor of anthropology at John Jay College of Criminal Justice and expert on urban redevelopment, was skeptical that the Area Median Income is an accurate indicator of the wealth of a community.

“When they are argued in public forums they sound official,” Snajdr said. “But when people walk home and try to pay their rent, pay their bills and they get laid off from their jobs, I don’t know that those measurements really mean a lot.”

Yonah Lieberman, an activist with Metropolitan Council on Housing, a tenant union, said there wasn’t a good yardstick. “We have to face the fact that this system has failed to give a satisfactory definition of what’s affordable,” he said.

There have been discussions at the City Council to bend the traditional standards for calculating the AMI to create a more community-sensitive value that could reflect the actual level of wealth of each neighborhood, Council member Laurie Cumbo said during a town hall meeting. – Marco Poggio