As a result, the “significant underutilization of labor resources” that Fed officials highlighted last month as they renewed a pledge to keep interest rates low for a “considerable period” is probably even more severe than currently estimated. And the information gap means policy makers may have more difficulty gauging the right moment to raise rates off zero.

“We have more slack than the official statistics suggest,” said Michelle Meyer, a senior U.S. economist at Bank of America Corp. in New York. “Because it’s difficult to measure underutilization, there’s still a lot of uncertainty as to how much slack remains, which means there’s uncertainty as to the appropriate stance of monetary policy.”

The Labor Department can put its finger on how many people are working part-time because full-time jobs aren’t available, or how many are so discouraged that they’re not even looking for employment. Other forms of underemployment -- for example the graduate with an English degree who’s working as a barista --are harder to pinpoint though just as important in trying to measure whether the labor market has improved.