Productivity is probably the most important measure of economic health that policy makers know the least about.

Its pace will help determine how soon Federal Reserve Chair Janet Yellen and her colleagues increase interest rates and how far rates will rise.

A quicker advance would argue for a later lift-off because the economy would have more room to run before bumping up against capacity constraints. It also eventually would require a higher ending point to prevent the more-vibrant expansion from overheating. Slower productivity would call for the opposite strategy.

The trouble, according to former Fed Vice Chairman Alan Blinder, is that economists — including those at the Fed — don't have a good idea of how fast productivity will grow in the next few years.

The long-run trend is "hugely important," but "it can take years" to recognize any changes, he said.

Yellen will lay out the central bank's views of the economy and policy when she testifies before Congress next week. At their Jan. 27-28 meeting, officials discussed the timing and pace of potential rate increases. Many were inclined to keep the benchmark federal fund rate near zero "for a longer time," according to minutes of the gathering.

John Fernald, a senior research adviser at the Federal Reserve Bank of San Francisco, pegs the trend growth rate of productivity at 1.8 percent a year for U.S. businesses and 2.1 percent for the economy.

The margin of error around Fernald's forecast is wide, as he is the first to acknowledge.

"There's basically an 80 percent chance over the next 10 years that productivity growth will average between 1 and 3 percent," the Fed official said.

Productivity measures the efficiency of the economy and matters beyond the central bank. It governs how fast the economy can expand, how much companies can earn and pay their workers, and how much the government can increase its budget.