U.S. home prices slid further down in December, concluding a quarter that saw an 18.2% drop in home prices from year-ago levels, according to a measure published by Standard & Poor’s.

The S&P Case-Shiller U.S. National Home Price Index, a measure that covers all nine U.S. census divisions, recorded an 18.2% decline in the fourth quarter of 2008 from the fourth quarter of 2007. This is the largest drop in the index’s 21-year history.

S&P's Case-Shiller 10-City composite measure showed a drop of 19.2% in home prices and its 20-City composite measure showed a drop of 18.5% in home prices.

“If one looks in detail at the annual return data, it can be seen that 13 of 20 MSAs and the two composites have been reporting consecutive declines since December 2007,” David Blitzer, chairman of S&P’s index committee, said in prepared remarks. “The monthly data follows a similar trend, with all of the metro areas reporting at least four consecutive months of negative returns.”

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