FACTBOX:The previous 10 S&P 500 bear markets

NEW YORK (Reuters) - The S&P 500 index .SPX plunged into a bear market on Wednesday -- more than 20 percent below its record high close of 1,565.15 points on October 9, 2007 -- after ceding to the pressure of a housing slump, a credit crisis, record-high oil prices and a weakening economy.

The S&P 500 was officially introduced in 1957 but its value has been extrapolated. Since 1929, whenever the index has fallen into a bear market, it has on average shed 29.4 percent of its value for the duration of the slump, which has averaged just over a year.

The S&P 500's worst bear market occurred in the early years of the Great Depression and stretched from April 10, 1930, to June 1, 1932.

The following is a recap of the previous 10 bear markets for the S&P 500, using "The Stock Traders Almanac 2008" data:

*January 4, 2002 to October 9, 2002

S&P 500 percentage loss: 33.8 percent

Number of days it lasted: 278

What happened? WorldCom and Enron's accounting fraud sparked fears that more firms could be artificially boosting their bottom lines.

*March 24, 2000 to September 21, 2001

S&P 500 percentage loss: 36.83 percent

Number of days it lasted: 546

What happened? The bursting of the dot-com bubble, following a period of soaring stock prices and exuberant speculation on new Internet companies. The United States entered a brief recession in March 2001.

July 17, 1998 to August 31, 1998

S&P 500 percentage loss: 19.3 percent

Number of days it lasted: 45 -- the shortest bear market on record.

What happened? Hedge fund Long Term Capital Management collapsed, requiring a rescue organized by the U.S. Federal Reserve.