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.

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.

Presentation of index returns

Some indices, such as the S&P 500 Index, have multiple versions. These versions can differ based on how the index components are weighted and on how dividends are accounted. For example, there are three versions of the S&P 500 Index: price return, which only considers the price of the components, total return, which accounts for dividend reinvestment, and net total return, which accounts for dividend reinvestment after the deduction of a withholding tax. The Wilshire 4500 and Wilshire 5000 indices have five versions each: full capitalization total return, full capitalization price, float-adjusted total return, float-adjusted price, and equal weight. The difference between the full capitalization, float-adjusted, and equal weight versions is in how index components are weighted.

Criticism of capitalization-weighting

One argument for capitalization weighting is that investors must, in aggregate, hold a capitalization-weighted portfolio anyway. This then gives the average return for all investors; if some investors do worse, other investors must do better (excluding costs).

Indices and passive investment management

Passive management is an investing strategy involving investing in index funds, which are structured as mutual funds or exchange-traded funds that track market indices. The SPIVA (S&P Indices vs. Active) annual "U.S. Scorecard", which measures the performance of indices versus actively managed mutual funds, finds the vast majority of active management mutual funds underperform their benchmarks, such as the S&P 500 Index, after fees. Unlike a mutual fund, which is priced daily, an exchange-traded fund is priced continuously and is optionable.

Ethical stock market indices

Several indices are based on ethical investing, and include only companies that meet certain ecological or social criteria, such as the Calvert Social Index, Domini 400 Social Index, FTSE4Good Index, Dow Jones Sustainability Index, STOXX Global ESG Leaders Index, several Standard Ethics Aei indices, and the Wilderhill Clean Energy Index. Other ethical stock market indices may be based on diversity weighting (Fernholz, Garvy, and Hannon 1998). In 2010, the Organization of Islamic Cooperation announced the initiation of a stock index that complies with Sharia's ban on alcohol, tobacco and gambling. Critics of such initiatives argue that many firms satisfy mechanical "ethical criteria" (e.g. regarding board composition or hiring practices) but fail to perform ethically with respect to shareholders (e.g. Enron). Indeed, the seeming "seal of approval" of an ethical index may put investors more at ease, enabling scams. One response to these criticisms is that trust in the corporate management, index criteria, fund or index manager, and securities regulator, can never be replaced by mechanical means, so "market transparency" and "disclosure" are the only long-term-effective paths to fair markets. From a financial perspective, it is not obvious whether ethical indices or ethical funds will out-perform their more conventional counterparts. Theory might suggest that returns would be lower since the investible universe is artificially reduced and with it portfolio efficiency. (It conflicts with the Capital Asset Pricing Model, see above.) On the other hand, companies with good social performances might be better run, have more committed workers and customers, and be less likely to suffer reputation damage from incidents (oil spillages, industrial tribunals, etc.) and this might result in lower share price volatility, although such features, at least in theory, will have already been factored into the market price of the stock. The empirical evidence on the performance of ethical funds and of ethical firms versus their mainstream comparators is very mixed for both stock and debt markets.