Indeed, lately, some of the very scholars and policymakers who once put Nairu at the center of their economic analysis have lost faith that they have a good handle on it at all. The number is shaped by such factors as the technology people use to match up with new employers and people’s willingness to relocate to find work. But like many concepts in economics, it can’t be calculated directly. Rather, it must be inferred based on what is happening to other variables, which inevitably involves guesswork, assumptions and the use of historical data in a changing world.

“It’s not a terribly useful tool right now,” said Alan Blinder, a Princeton economist and former vice chairman of the Federal Reserve. “For it to be useful you have to have at least a little confidence you know the number. You don’t need to know it to two decimal places, but within a reasonable range. If your range is 2.5 to 7, that doesn’t tell you anything.”

In the mid-1990s, when Mr. Blinder was at the Fed, he and Janet Yellen, then a Fed governor, tried to persuade the chairman Alan Greenspan that interest rate increases were needed because the unemployment rate was quickly falling below estimates of Nairu in the 6 percent and higher range.

Mr. Greenspan won the argument, as he almost always did at the Fed in that era. And with hindsight it seems he was correct. Unemployment kept falling through the late 1990s, and reached as low as 3.8 percent in the spring of 2000, without evident flares of inflation pressure.

Indeed, the jobless rate in the United States has now fallen below 4.5 percent just three times since 1970 — the late 1990s, 2006-2007, and in the last year. In none of those times did inflation flare up (and in the current expansion, it hasn’t flared up yet).

Looking abroad is instructive, too. Each country has a distinct labor market that probably implies a different natural rate of unemployment. But it is striking that in Germany and Japan, joblessness levels have fallen in the last couple of years below estimates that officials in those countries have generally considered their Nairu — without inflation taking hold.

That tells some officials here, charged with making policy in 2018, that the sensible strategy is to see just how low joblessness can go without causing problems.