Stock Market Index

A measure of the change in the share prices of different companies

What is a Stock Market Index?

A stock market index, also known as a stock index, measures a section of the stock market. In other words, the index measures the change in the share prices of different companies.

Stock Market Index

The stock index is determined by calculating the prices of certain stocks (generally a weighted average). It is a tool widely used by financial institutions and investors to compare the return on specific investments and to describe the market.


  • A stock market index measures a section of the stock exchange.
  • It is determined by calculating the prices of certain stocks.
  • Three of the most popular stock market indices in the USA are S&P 500, Dow Jones Industrial Average (DJIA), and Nasdaq Composite.

Types of Stock Market Indices

Stock market indices may be classified in different ways. A “global” or “world” stock market index, such as the MSCI World or the S&P Global 100, contains stocks from multiple regions. Regions can be defined geographically (for example, Asia, Europe) or by levels of income or industrialization (for example, frontier markets, developed markets).

A national index represents the performance of the stock market of a definite country and reflects the sentiment of investors on the state of the economy. National indices include the stocks of large companies listed on the nation’s largest stock exchanges. Many indices are regional, e.g., the FTSE Developed Asia Pacific Index or the FTSE Developed Europe Index.

There are more specialized indices that track the performance of specific sectors of the stock market. In the United States, specialized indices include the Morgan Stanley Biotech Index, which consists of 36 American companies in the biotechnology industry, and the Wilshire US REIT, which tracks more than 80 U.S. real estate investment trusts. There are other indices that may monitor organizations of a certain size or type of management.

The Importance of Indices

The daily results of stock market indices are perhaps the most popular and significant numbers in the whole world of investing and finance. Probably the world’s best-known and most widely used stock market index, the Dow Jones Industrial Average (DJIA) consists of 30 largest traded companies in the United States.

Many investors use market indices for managing their investment portfolios and following the financial markets. Indexes are deeply integrated into the investment management business, and funds use them as benchmarks for performance comparisons.

Indices as Benchmarks

Indexes serve as benchmarks for different purposes in the financial markets. As mentioned, the Dow Jones Industrial Average, Nasdaq Composite, and S&P 500 are the three most popular U.S. indexes. The three indexes contain the 30 largest stocks in the U.S. by market capitalization, all stocks on the Nasdaq Exchange, and the 500 largest stocks, respectively. Benchmarks can be a good indicator of the overall U.S. stock market since they include some of the most valuable U.S. stocks.

Investors can also use performance and benchmark values to follow investments by segments. Some investors may diversify their investment portfolios based on the returns or expected returns of certain segments. Furthermore, a specific index may act as a benchmark for a mutual fund or a portfolio.

Additional Resources

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