"Standard & Poor's introduced its first stock index in 1923. Before 1957, its primary daily stock market index was the "S&P 90", a value-weighted index based on 90 stocks. Standard & Poor's also published a weekly index of 423 companies. The S&P 500 index in its present form began on March 4, 1957. Thanks to the computer technology emerging at the time, this index could be calculated and disseminated in real time. The S&P 500 is widely employed as a measure of the general level of stock prices, as it includes both growth stocks and the generally less volatile value stocks."
