All of the companies in the list are large publicly-held companies which trade on major US stock exchanges such as the New York Stock Exchange and Nasdaq. The market-value weighted performance of the stocks of these companies is known as the S&P 500 index. After the Dow Jones Industrial Average, the S&P 500 is the most widely-watched index of large-cap US stocks and is considered to be a bellwether for the US economy.
Many index funds and exchange traded funds track the performance of the S&P 500 by holding the same stocks as the S&P 500 index, attempting to match its performance. Partly because of this, a company which has its stock added to the list may see a boost in its stock price as mutual fund managers are forced to purchase that company's stock in order to match their index funds' composition to that of the S&P 500 index.
In stock and mutual fund performance charts, the S&P 500 index is often used as a baseline for comparison. The chart will show the S&P 500 index, with the performance of the target stock or fund overlaid.
Although the 500 companies in the list are among the largest in the US, it is not simply a list of the 500 biggest companies. The companies are carefully selected to ensure that they are representative of various industries in the US economy. In addition, companies which are privately held and stocks which do not have sufficient liquidity are not in the index. By contrast, the Fortune 500 attempts to list the 500 largest companies in the United States, regardless of whether they are publicly or privately held and without adjustment for industry representation.
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