In finance, a stock index, or stock market index, is an index that measures the performance of a stock market, or of a subset of a stock market. It helps investors compare current stock price levels with past prices to calculate market performance.
Two of the primary criteria of an index are that it is investable and transparent: The methods of its construction are specified. Investors may be able to invest in a stock market index by buying an index fund, which is structured as either a mutual fund or an exchange-traded fund, and "track" an index. The difference between an index fund's performance and the index, if any, is called tracking error.
Types of indices by coverage
Stock market indices may be classified and segmented by the set of underlying stocks included in the index, sometimes referred to as the "coverage". The underlying stocks are typically grouped together based on their underlying economics or underlying investor demand that the index is seeking to represent or track. For example, a 'world' or 'global' stock market index—such as the MSCI World or the S&P Global 100—includes stocks from all over the world, and satisfies investor demand for an index for broad global stocks. Regional indices that make up the MSCI World index, such as the MSCI Emerging Markets index, include stocks from countries with a similar level of economic development, which satisfies the investor demand for an index for emerging market stocks that may share similar economic fundamentals. The coverage of a stock market index is separate from the weighting method. For example, the S&P 500 market-cap weighted index covers the 500 largest stocks from the S&P Total Market Index, but an equally weighted S&P 500 index is also available with the same coverage.
Global coverage These indices attempt to represent the performance of the global stock market. For example, the FTSE Global Equity Index Series includes over 16,000 companies. Regional coverage These indices represent the performance of the stock market of a single geographical region. Some examples of these indices are the FTSE Developed Europe Index, and the FTSE Developed Asia Pacific Index. Country coverage These indices represent the performance of the stock market of a single country—and by proxy, reflects investor sentiment on the state of its economy. The most frequently quoted market indices are national indices composed of the stocks of large companies listed on a nation's largest stock exchanges, such as the S&P 500 Index in the United States, the Nikkei 225 in Japan, the DAX in Germany, the NIFTY 50 in India, and the FTSE 100 in the United Kingdom. Exchange-based coverage These indices may be based on the exchange on which the stocks are traded, such as the NASDAQ-100, or groups of exchanges, such as the Euronext 100 or OMX Nordic 40. Sector-based coverage These indices track the performance of specific market sectors. Some examples are the Wilshire US REIT Index, which tracks more than 80 real estate investment trusts, and the NASDAQ Biotechnology Index which consists of about 200 firms in the biotechnology industry.
Types of indices by weighting method
Stock market indices may be categorized by their index weight methodology, or the rules on how stocks are allocated in the index, independent of its stock coverage. For example, the S&P 500 and the S&P 500 Equal Weight each cover the same group of stocks, but the S&P 500 is weighted by market capitalization, while the S&P 500 Equal Weight places equal weight on each constituent. Some common index weighting methods are listed below. In practice, many indices will impose constraints, such as concentration limits, on these rules.