As of 2015, the S&P 500 Index returns an average of 9.7 percent annually, according to MarketWatch, but individual 10-year periods vary widely in their performance, ranging from negative 0.9 percent in the 2000s to 19.4 percent during the 1950s. The one-year rate of return as of Sept. 25, 2015, is negative 1.76 percent, as reported by S&P Dow Jones Indices LLC.
The S&P 500 Index's average annual return for the years 1930 to 2013 was 9.7 percent, according to MarketWatch. Returns for 10-year periods within the 1930 to 2013 range varied, with returns for three of the included decades above 18 percent annually and three of the decades returning between 5.9 percent and 9.2 percent. The index's returns were slightly negative during the 2000s and the 1930s.
The S&P 500 Index contains 502 total companies as of Aug. 31, 2015, according to S&P Dow Jones Indices LLC. The largest companies in the index are Apple, Microsoft and Exxon Mobil, and the information technology, health care and finance sectors make up more than 50 percent of the total index holdings.
Only U.S. companies with more than half of their stock available for sale and a stock price above $1 per share are eligible for inclusion in the S&P 500 Index, according to Kimberly Amadeo for About.com. The index tracks large cap stocks, and S&P 500 companies must have market capitalizations over $5.3 billion.