Price-Weighted Index
A price-weighted index calculates its level by averaging the share prices of its constituent stocks, so that higher-priced shares carry greater weight regardless of company size, total market value, or shares outstanding — a methodology that dominated early U.S. index design but is now considered an anomaly among modern benchmarks.
The Dow Jones Industrial Average is the world's most famous price-weighted index and the longest continuously published U.S. stock market gauge, tracing its origins to Charles Dow's 1896 average of twelve industrial stocks. The Nikkei 225, Japan's benchmark large-cap index, is also price-weighted. Both indices weight constituents purely by their nominal share price rather than by any measure of company value or size.
The mechanics of price weighting produce counterintuitive distortions. A stock trading at $500 per share has five times the index influence of a $100 stock regardless of the companies' relative sizes. A company that executes a 2-for-1 stock split — halving its share price without changing its fundamental value — instantly loses half its index weight, which is why the Dow Jones Industrial Average uses a divisor that is continuously adjusted for splits, spinoffs, and stock dividends to maintain index continuity.
This divisor, known as the Dow Divisor, is publicly published by S&P Dow Jones Indices. As of recent years it stands at roughly 0.15, meaning that a $1 move in any DJIA constituent moves the index by approximately $6.67 points. The divisor must be adjusted whenever a constituent undergoes a corporate action that changes its price without reflecting an actual market value change, a mechanical complication that cap-weighted indices avoid entirely.
Price weighting also introduces an inherent anti-diversification bias. Because the index level is dominated by the handful of highest-priced stocks, a small number of constituents drive most of the daily return. In the DJIA, three or four stocks have historically accounted for a disproportionate fraction of the index's sensitivity to market moves.
Modern index construction has largely abandoned price weighting in favor of market-cap weighting or factor-based alternatives, precisely because share price is a function of both value and corporate decisions about share count that have no economic meaning for investors. However, the DJIA retains cultural and media significance as a shorthand for 'the market' in public discourse, and understanding its price-weighted construction is essential for interpreting why it sometimes diverges from cap-weighted indices like the S&P 500 during periods when high-priced or low-priced stocks move sharply.