(Bloomberg News) A decline in unemployment and pickup in manufacturing point to accelerating U.S. growth. Some economists say the numbers may not be as good as they look.

One reason: The severity of the economy's plunge in late 2008 and early 2009 after Lehman Brothers Holdings Inc. collapsed threw a wrench into models used to smooth the data for seasonal changes, according to analysts at Goldman Sachs Group Inc. and Nomura Securities International Inc.

The jobless rate has dropped 0.4 percentage point over the past two months, according to the Labor Department, and the Institute for Supply Management's factory index has climbed more than three points since the end of August. Signs the world's largest economy was strengthening helped propel a 14 percent gain in the Standard & Poor's 500 Index in the past eight weeks.

"The impact of the financial crisis does seem to have affected seasonal factors for several indicators," Andrew Tilton, a senior economist at Goldman Sachs, said in a telephone interview from New York. It "might tend to make things look a little better in the early winter and look a little worse in the spring time."

Most economic data are adjusted for seasonal changes to facilitate month-to-month comparisons. Without those changes, for example, construction would always pick up in the summer, when the weather is milder, and decline in the winter.

The adjustment process in unable to distinguish between a one-time shock, like Lehman's demise, and a recurring issue that would need to be smoothed away. For that reason, the mechanism gives some data a leg up from about September through about March before turning negative the rest of the year.

The economy contracted at an average 7.8 percent annual pace from October 2008 through March 2009, the worst back-to- back quarters in the post World War II era. The 18-month recession ended in June 2009.

The adjustment process "has been knocked out of whack by the financial crisis," Ellen Zentner, a senior U.S. economist at Nomura in New York, said in a telephone interview. "The model ends up adjusting for a growth pattern that isn't there. The sudden drop-off in economic activity in late 2008 is not a pattern, it doesn't happen late every year. It was a one-off event."

"Around April the seasonal bias turns and starts working against the data," said Zentner, who shared her findings in research notes issued in December and last week.

Nomura was the fourth-best employment forecaster for the two years through December, according to Bloomberg calculations. Goldman Sachs was No. 1 among forecasters of gross domestic product during the 12 months through June 2009.

Stocks were little changed today as European finance ministers gathered in Brussels to discuss new budget rules and a Greek debt swap. The Standard & Poor's 500 Index rose 0.1 percent to 1,317 at 9:40 a.m. in New York.

French business confidence unexpectedly fell in January to the lowest in almost two years, providing the latest sign that Europe's second-largest economy is mired in a recession, figures from the statistics office Insee showed today in Paris.

Elsewhere, prices paid by Australian producers decelerated in the October through December period for a third straight quarter, boosting scope for the central bank to lower borrowing costs next month, data from the Bureau of Statistics showed in Sydney.

The U.S. seasonal distortions are most acute for the jobless rate, according to a report by Tilton issued Jan. 13. The shift moves the rate by about a tenth of a point per month on average relative to the adjustment before the crisis, he said. The influence is most positive from November through January, Tilton's research showed.

Jobless Rate