WASHINGTON - Concerns are rising that the economy is at risk of slipping into a "double-dip" recession.
So what exactly is a double-dip recession? Robert Hall has an idea of what one looks like but no precise definition. He's chairman of the National Bureau of Economic Research, a group of academic economists that officially declares the starts and ends of recessions.
In Hall's view, a double dip is akin to a continuous recession that's punctuated by a period of growth, then followed by a further decline in the economy.
In econo-speak, Hall explains: "The idea - hypothetical because it has yet to happen - is that activity might rise for a period, but not far enough to complete a cycle, then fall again, and finally rise above its original level, only then completing the cycle."
Hall says the closest the United States has come to a double dip was in 1980 and 1981. But the NBER concluded that those were two distinct, though closely spaced, recessions - "not a double dip," he says.
Not so, says Sung Won Sohn, professor at California State University, Channel Islands. Sohn says the back-to-back recessions of the early '80s fit his definition of a double dip: a recession followed by a short period of growth followed by a recession.
"There is no mathematical formula; it's a judgment call," Bethune says.
The NBER has declared the economy fell into a recession in December 2007. It hasn't yet pinpointed an end to the recession. It said in April that it would be "premature" to do so.
Many other economists say the recession ended in June or July of last year. The economy returned to growth again in the third quarter of 2009, after four straight quarters of declines. More recently, the economy has added jobs in each of the first five months of this year.
Still, the threats to the recovery from overseas and at home are growing. So are the risks that the recovery will fade out.
Economists say the odds of that remain low but have crept up since a couple of months ago. Analysts are downgrading their growth forecasts for the second half of this year.
In determining the starts and stops of recessions, the NBER reviews data that make up the nation's gross domestic product. The GDP measures the value of goods and services produced in the United States. The NBER also reviews incomes, employment and industrial activity.
The panel, based in Cambridge, Mass., tends to take its time in declaring when a recession has started or ended.
It announced in December 2008 that the recession had actually started one year earlier - in December 2007.
And it declared in July 2003 that the 2001 recession was over.
It had actually ended 20 months earlier - in November 2001.
In President George W. Bush's eight years in office, the United States fell into two recessions. The first started in March 2001 and ended that November. The second started in December 2007; its end date is pending the NBER's determination.
The timing of the NBER's decision likely means little to ordinary Americans now muddling through a sluggish economic recovery and weak job market.
Many will continue to struggle. Unemployment usually keeps rising well after a recession ends. After the 2001 recession, for instance, unemployment didn't peak until June 2003 - 19 months later.