Dow Jones Industrial Average
The Dow Jones Industrial Average is a price-weighted average of 30 large US companies and one of the oldest gauges of the American stock market. Each member’s influence depends on its share price rather than its market value, so the average measures something quite different from what the nightly headlines imply.
The math
Take two members of a price-weighted average. Stock A trades at $500, Stock B at $50.
Both fall 5 percent on the same day: A subtracts $25 from the price sum, B subtracts $2.50. A moves the average ten times as much, even if B’s company is worth far more in total.
Now suppose the 30 share prices sum to $5,000. A represents 10 percent of that sum, so its 5 percent drop pulls the average down 0.5 percent: a $500 hit on a $100,000 tracking position.
B’s identical percentage drop costs the same investor $50.
| Stock A | Stock B | |
|---|---|---|
| Share price | $500 | $50 |
| Same 5% drop, in points | $25 | $2.50 |
| Hit to a $100,000 tracker | $500 | $50 |
Same decline, tenfold difference, purely because of where each share price happens to sit.
The trap
Point headlines. A drop of several hundred points sounds dramatic, yet the same point move means a different percentage at every level of the average, and a stock split silently rewires a member’s influence without changing its business at all.
Investors who make decisions off “the Dow plunged today” are reacting to an arithmetic artifact from the 1890s.
The move
Translate every Dow figure into a percentage before letting it register, and benchmark real portfolios against a cap-weighted index like the S&P 500 instead. For a stock picker the Dow has one honest use: its 30 members form a shortlist of established franchises worth studying individually.
The average of their prices tells you almost nothing; the businesses underneath sometimes do.