The Bureau of Economic Analysis on Thursday revised away the contraction in gross domestic product in the first quarter, erasing an ugly patch in the ongoing economic recovery. It also marked down growth over the past few years, finding that average growth from the last quarter of 2011 to the first quarter of this year was just two percent, rather than the 2.2 percent previously estimated.

Here's why they did it:

The Bureau constantly revises its economic output statistics to try to better capture the complicated reality of the U.S. economy.

Each quarterly estimate is updated three times as new data is available. Every summer, the Bureau updates its previous estimates for the past several years to include new data sources and tweak its methodology for calculated gross domestic product. And every five years, it does a comprehensive overhaul of all of its GDP statistics going back to 1929.

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