A Primer on U.S. Stock Price Indices

T he measurement of the " average " price of common stocks is a matter of widespread interest. Investors want to know how " the market " is doing, and to be able to compare their returns with a meaningful benchmark. Money managers often have their compensation tied to performance, typically measured by comparing their results to a benchmark portfolio, so they and their clients are interested in the benchmark portfolio's returns. And policymakers want to judge the potential for sudden adjustments in stock prices when differences from " fundamental value " emerge. The most widely quoted stock price index, the Dow Jones Industrial Average, has been supplemented by other popular indices that are constructed in a different way and pose fewer problems as a measure of stock prices. At present, a number of stock price indices are reported by the few companies that we will consider in this paper. Each of these indices is intended to be a benchmark portfolio for a different segment of the universe of common stocks. This paper discusses some of the issues in constructing and interpreting stock price indices. It focuses on the most widely used indices: the Dow Jones Industrial Average, the Standard & Poor's 500, the Russell 2000, the NASDAQ Composite, and the Wilshire 5000. The first section of this study addresses issues of construction and interpretation of stock price indices. The second section compares the movements of the five indices in the last two decades and investigates the relationship between the returns on the reported indices and the return on " the market. " Our results suggest that the Dow Jones Industrial Average (Dow 30) has inherent problems in its construction. Even so, while it overstates the performance of its segment of the stock market during the 1990s, adding to the popular impression that the blue chips have been the star performers, it does a fairly good job of reflecting the performance of large companies. We also find that the five indices that we examine