A decline in unemployment and pickup in manufacturing point to accelerating U.S. growth. However, some economists say the numbers may not be as good as they look, Bloomberg reported.

One reason: The severity of the economy’s plunge in late 2008 and early 2009 after Lehman Bros. 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 impact of the financial crisis does seem to have affected seasonal factors for several indicators,” Andrew Tilton, a senior economist at Goldman Sachs, told Bloomberg. It “might tend to make things look a little better in the early winter and look a little worse in the springtime.”

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 is 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.