The Standard & Poor's 500 index was created on March 4, 1957, and is considered to be one of the leading benchmarks for the U.S. stock market. As of 2015, a significant number of investors use the S&P 500 index to measure the performance of their own stock portfolios.
According to a report on the Chicago Tribune, approximately $5.58 trillion are benchmarked against the S&P 500 index in 2013. The precursor to the S&P 500 index were two indexes developed by Standard & Poor's in 1923 and 1926. The first index measured the stocks of 233 companies on a weekly basis.