WASHINGTON (MarketWatch) — Productivity, a sore spot for the U.S. economy over the past few years, has now declined in three straight quarters, according to data released Tuesday.

Productivity in the second quarter unexpectedly fell 0.5%, well below expectations, the Labor Department said. Economists surveyed by MarketWatch had forecast a 0.3% gain in productivity in the quarter.

Productivity is down 0.4% from a year earlier, the first year-over-year decline since the second quarter of 2013.

Output of goods and services increased at a 1.2% clip in the second quarter. Hours worked rose 1.8%.

Productivity measures how much an employee produces in an hour of work. Higher productivity is regarded as the key to a rising standard of living over time because it tends to lead to higher pay for workers and larger profits for companies.

The average annual rate of productivity growth from 2007 to 2015 has sunk to 1.3%, well below the long-term rate of 2.2% per year from 1947 to 2014.

There is widespread debate among economists about the causes of weak productivity. Some blame the lack of capital investment. Some question the government’s measuring skills.

Earlier this summer, Federal Reserve Chairwoman Janet Yellen gingerly pressed Congress to focus on ways to boost “very low” labor productivity.

“I am not going to give you detailed instruction, this is up to Congress to decide,” Yellen said.

But public investment, workforce development and the pace of technological progress are widely seen by economists as factors that could influence productivity and “should be on Congress’ list to focus on,” she said.

In the second quarter, unit-labor costs remained well contained. They rose 2.0% in the second quarter and are up a mild 2,1% in the past 12 months. Unit labor costs in the first quarter were revised sharply lower to a 0.2% decline from the prior estimate of a 4.5% gain.

Meanwhile, hourly compensation for all workers rose at a 1.5% rate in the second quarter, though it was down 1.1% adjusted for inflation.