An index is a statistical measure of the changes in a portfolio of stocks representing a part of the whole market.
It would be too difficult to track every single security trading in the country. To get around this, we take a smaller sample of the market that is representative of the whole. Thus, just as pollsters use political surveys to gauge the sentiment of the population, investors use indexes to track the performance of the stock market. Ideally, a change in the price of an index represents an exactly proportional change in the stocks includedin the index.
Mr. Charles Dow created the first and, consequently, most commonly known index back in May of 1896. At that time, the Dow index contained 12 of the largest public companies in the U.S. Today, the Dow Jones Industrial Average (DJIA) contains 30 of the largest and most influential companies in the U.S.
Before the digital age, calculating the price of a stock market index had to be kept as simple as possible. The original DJIA was calculated by adding up the prices of the 12 companies and then dividing that number by 12. These calculations made the index truly nothing more than an average, but it served its purpose.
Today, the DJIA uses a slightly different methodology, called price-based weighting. In this system, the weight of each security is the stock's price relative to the sum of all the stock prices. The trouble with price-based weighting is that a stock split changes the weight of a company in the index, even though there is no fundamental change in the business. Because of this, not too many indexes are weighted on price.
Most indexes weigh companies determined by market capitalization. If a company's market cap is $1,000,000 and the value of all stocks in the index is $100,000,000, then the company would be worth 1% of the index. These types of systems are made possible by computers - most are calculated to the minute, so they are very accurate reflections of the market.
It's significant to note that an index is nothing more than a list of stocks; anybody can create one. This was especially true during the dotcom bull market, when practically every publication created an index representing a section of new economy stocks. What sets the big indexes apart from the small ones is the status of the company that puts out the index.