An index is a statistical means of calculating a change in an economy or market. The index is typically a weighted average representative sample of the market. There are numerous lists of indices accessible for exchanging, measuring the performance of the share trading system of different markets. Some examples include, the Standard & Poor's 500, one of the world's best known indices and one of the most commonly used benchmarks for the stock market. S&P500 includes 70% of the total stocks traded in the United States. Additionally, the Dow Jones Industrial Average (DJIA) is also a very well-known index, but it only represents stock values from 30 of the nation's publicly traded companies.