A stock market index is a method of measuring a section of the stock market. Many indices are cited by news or financial services firms and are used as benchmarks, to measure the performance of portfolios such as mutual funds. Types of indices
Stock market indices may be classed in many ways. A 'world' or 'global' stock market index includes (typically large) companies without regard for where they are domiciled or traded. Two examples are MSCI World and S&P Global 100. A national index represents the performance of the stock market of a given nation—and by proxy, reflects investor sentiment on the state of its economy. The most regularly quoted market indices are national indices composed of the stocks of large companies listed on a nation's largest stock exchanges, such as the American S&P 500, the Japanese Nikkei 225, and the British FTSE 100. The concept may be extended well beyond an exchange. The Wilshire 5000 Index, the original total market index, represents the stocks of nearly every publicly traded company in the United States, including all U.S. stocks traded on the New York Stock Exchange (but not ADRs or limited partnerships), NASDAQ and American Stock Exchange. Russell Investment Group added to the family of indices by launching the Russell Global Index. More specialised indices exist tracking the performance of specific sectors of the market. Some examples include the Wilshire US REIT which tracks more than 80 American Real Estate Investment Trusts and the Morgan Stanley Biotech Index which consists of 36 American firms in the biotechnology industry. Other indices may track companies of a certain size, a certain type of management, or even more specialized criteria — one index published by Linux Weekly News tracks stocks of companies that sell products and services based on the Linux operating environment.  Index versions
Some indices, such as the S&P 500, have multiple versions. These versions can differ based on how the index components are weighted and on how dividends are accounted for. 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. As another example, 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.  Weighting
An index may also be classified according to the method used to determine its price. In a price-weighted index such as the Dow Jones Industrial Average, Amex Major Market Index, and the NYSE ARCA Tech 100 Index, the price of each component stock is the only consideration when determining the value of the index. Thus, price movement of even a single security will heavily influence the value of the index even though the dollar shift is less significant in a relatively highly valued issue, and moreover ignoring the relative size of the company as a whole. In contrast, a market-value weighted or capitalization-weighted index such as the Hang Seng Index factors in the size of the company. Thus, a relatively small shift in the price of a large company will heavily influence the value of the index. In a market-share weighted index, price is weighted relative to the number of shares, rather than their total value. Traditionally, capitalization- or share-weighted indices all had a full weighting, i.e. all outstanding shares were included. Recently, many of them have changed to a float-adjusted weighting which helps indexing. A modified capitalization-weighted index is a hybrid between capitalization weighting and equal weighting. It is similar to a capitalization...
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