By aggregating the value of a related group of stocks or other investment vehicles together and expressing their total values against a base value from a specific date. Market indexes help to represent an entire stock market and thus give investors a way to monitor the market’s changes over time.
Index values are used by investors to track changes in the stock market over long periods of time. For example, the widely used S&P 500 Index is computed by combining 500 large-cap U.S. stocks together into one index value. Investors can track changes in the index’s value over time and use it as a benchmark against which to compare their own portfolio returns.