Despite preliminary signs of a turnaround in the U.S. economy, the National Bureau of Economic Research isn't about to declare the worst over. "The data continue to show substantial declines in real activity in manufacturing, the sector reflected in the industrial production index, and in real manufacturing and trade sales," the NBER said on its Web site. "Aggregate employment has fallen substantially as well." Back in November, the NBER announced that the economy had slipped into a recession in March, ending 10 years of economic expansion. But since then, investors have seen a slew of reports indicating that the worst may be over. Fourth-quarter gross domestic product rose an unexpected 0.2% amid strong government and consumer spending, according to a preliminary report. Meanwhile, nonfarm payrolls fell by just 89,000 in January, the smallest one-month job loss in six months. Manufacturing hit its highest point in a year and a half and the Leading Economic Index, or LEI, rose in January to levels unseen since August. The NBER uses four key measurements to determine the health of the economy: employment, industrial production, wholesale/retail trade and income. Of these, only income has behaved differently from previous recessions, largely because of declining import prices, especially oil, and the continuation of productivity growth, the panel said. "Despite the substantial decline in employment throughout the economy -- equal in percentage terms to the declines seen in previous recessions -- output fell less than is typical in a recession because of unusual productivity growth," the committee added. Although it's possible the highly regarded six-member panel believes the economy is on the road to recovery, the committee said it usually waits "until a substantial period of expansion has elapsed before declaring that a turning point is a true trough." For example, the panel waited until December 1992 to announce that a trough had occurred in March 1991. "The committee usually waits until activity has surpassed its previous peak before determining a trough date," it said, adding that it will not issue any judgment about whether the economy has bottomed "until it makes its formal decision on this point."