The response rate of the Labor Department’s monthly jobs survey is far higher (about 89 percent) than that of a political poll, but it has also fallen (from 96 percent in the 1980s). Not surprisingly, the people who do not respond have different experiences in the job market than those who do.

The trouble with the unemployment rate revolves around a technical concept known as “rotation-group bias,” explain the paper’s three authors, Mr. Krueger, Alexandre Mas and Xiaotong Niu. The government surveys people for four consecutive months, gives them eight months off and then surveys them for four more months. This pattern allows the Labor Department to track people’s experiences for more than a year in a way that is less burdensome than 16 months of monthly surveys would be.

Over time, the kinds of answers that people give — or the kinds of people who respond — change. In later months as part of the survey panel, people who aren’t working are less likely to report being available to work and having looked for a job in the previous four weeks, which is the definition for unemployment. The differences are big, too.

The unemployment rate in the first half of 2014 among people in the first month of being interviewed was 7.5 percent. Among people in the final month of being interviewed, it was only 6.1 percent. Because the Labor Department weights later panelists – for whom there is historical data – more heavily, the official unemployment rate during this period was 6.5 percent.

That number seems too low. The authors note that the higher jobless rate among early-month panelists correlates more strongly with some other economic indicators than the rate among later-month panelists.

If you’re tempted to blame President Obama for this situation, Jack Welch-style, you should dig into the data. The problem has existed, and been growing, for decades. A redesign of the survey in 1994, to move it from paper-based to computer-based, seems to be one cause. The full reasons aren’t clear, but something about the redesign seems to have changed the way people answer questions. The gap in jobless rates between early and later panelists starting spiking in 1994 and is now about twice as large as it was then.