NEW YORK (CNN/Money) - The research group that declares the start and end of U.S. economic recessions said Thursday that it saw some signs the latest recession may be over, but was not ready to formally declare that a new expansion had begun.

The National Bureau of Economic Research's business-cycle dating committee noted that payroll employment, its most important measure of economic activity, seemed to have stabilized in the first four months of 2002, pointing to the possibility that the recession that began in March 2001 is over.

"At some future date, the committee will determine the date of a trough in activity," it said in a statement. "The trough date will mark the end of the recession. The committee will not issue any judgment about whether the economy has reached a trough until it makes its formal decision on this point."

The NBER committee -- which includes one of the latest nominees for the Federal Reserve Board, Princeton economist Ben Bernanke -- defines a recession as "a significant decline in activity spread across the economy, lasting more than a few months, visible in industrial production, employment, real income, and wholesale-retail sales."

There is no official definition of a recession, though many people use two straight quarters of shrinking gross domestic product (GDP) as a rule of thumb. In the latest downturn, GDP contracted in only the third quarter, prompting some economists to question if there had been a recession at all.

Fed Chairman Alan Greenspan declared earlier this year that the recession was already over, and a 5.8-percent surge in GDP in the first quarter, following 1.7-percent growth in the fourth quarter of 2001, led many economists to agree.

The committee pointed out that it usually waits for several months before declaring a trough in economic activity -- it waited until December 1992 to declare that a trough had occurred in March 1991, for example.

The U.S. Labor Department reported last week that non-farm payrolls grew by 43,000 jobs in April, though the unemployment rate swelled to 6.0 percent. It was the first month jobs were added since July 2001, but the payroll data are also notoriously volatile, and most economists took April's gain with a grain of salt.

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Two other measures of economic strength watched by the NBER, including industrial production and personal income, have risen this year, while wholesale and retail sales have been volatile.