Dow Jones: Weighted by Share Price
The Dow Jones Industrial Average holds thirty companies weighted by their share price rather than their market value. That makes a high-priced share more influential than a larger company with a lower price, which is a construction almost no other index uses.
How it works
Thirty companies, and the weighting is the share price. Add the thirty prices together, divide by a figure that adjusts for splits and changes, and that is the index.
Almost every other major index weights by market value. Price weighting is an artefact of an era before computers, when adding thirty numbers and dividing was the practical limit of what could be published daily.
The consequence is immediate. A company whose shares trade at 300 has ten times the influence of one trading at 30, and share price is a function of how many shares exist rather than of how large the business is.
So a smaller company can outweigh a larger one. A business worth a fifth as much can move the index more, purely because it never split its stock.
The split problem
A split halves a company’s weight without changing anything. Two shares at 150 instead of one at 300 is identical ownership of an identical business, and the company’s influence on the index falls by half.
Membership is decided by a committee. There is no size threshold or objective criterion — a small group chooses which thirty companies represent American industry, and changes are announced rather than triggered.
It persists because everybody knows it. More than a century of continuous publication makes it the number people recognise, and recognition rather than construction is what keeps it in headlines. Nobody designing an index today would build it this way.
In practice
Funds tracking it exist and cost more than broad trackers. For thirty companies chosen by committee and weighted by an accident of share price, which is a difficult combination to justify on cost grounds.
Its futures still carry real volume. Familiarity produces liquidity, and liquidity makes an instrument tradeable regardless of whether its construction makes sense.
Over long periods it moves closely with broader indices. Thirty very large companies are correlated enough with the wider market that the construction matters less to the decade-long picture than to any single day.
One company can move it noticeably. With thirty constituents and price weighting, a gap in a single high-priced share shows up directly in the headline figure.
A stop on it exits a position in thirty companies at once, which is either what you intended or a very broad response to one company’s news.
Costs are unchanged by any of this. 2% of a median bar’s range per round trip on this history, whichever index is being traded.
Reading the headlines
Point moves are the specific trap. “The Dow fell 400 points” sounds dramatic and means very little without the level — at 40,000 that is one per cent, and at 10,000 it would have been four.
Percentage is the only comparable figure, both across time and against other indices. Convert the point move before reacting to it, and most of the drama in financial headlines about this particular index disappears immediately — which is a useful habit well beyond this one number.
What the Dow Jones is not
It is not weighted by company size. It is weighted by share price.
It is not rule-based. A committee picks the thirty.
It is not the American market. Thirty companies is not a market.
And it is not comparable in points. Only percentages compare.
When it fails as a measure
It fails hardest on any single day. Price weighting means one high-priced constituent can determine the direction of the number while most of the thirty went the other way.
A second failure is comparing point moves across decades. The same point change is a completely different percentage at different index levels.
A third is treating it as a benchmark. Thirty committee-selected companies is not a standard to measure a portfolio against.
A fourth is ignoring the split effect. Weights change on corporate actions that carry no information.
And a fifth is buying a tracker for it. Higher fees for a narrower and stranger index than the alternatives.
The deepest problem is that it cannot be replicated by an ordinary investor. A price-weighted index requires holding an equal number of shares in each company rather than an equal amount of money, which is the opposite of how anybody actually buys. Owning one share of each of the thirty is a portfolio nobody would deliberately construct.
And its long record is less continuous than it appears. Companies have been added and removed repeatedly, so the century-long chart is a chart of a changing list rather than of the same thirty businesses — every dropped constituent was dropped for a reason, and those reasons are absent from the history the number carries.
The original data
Of the 31,760 trading and investing videos in this site’s corpus, 6 have “dow jones” in the title at a
median of 6,250 views across 5 channels. “Nasdaq” returns 62 at a median of 4,718 across 42 channels, “S&P
500” returns 42 at a median of 9,664, and “index fund” returns 30 at a median of 74,230. The counts are in
research/corpus-coverage.json, produced by site/measure_corpus.py.
Six videos across the whole corpus is the smallest count of any major index here, against sixty-two for the Nasdaq and forty-two for the S&P 500. The index that dominates general news headlines is close to absent from the material people actually watch to learn about markets — which is a reasonable verdict on its usefulness. Treat it as a headline convention rather than a measurement, and convert every point move to a percentage before deciding it means anything.
Related
Nasdaq 100 is the other headline index and how its weighting works. Index funds explains why capitalisation weighting became the standard. And stock market is the wider introduction.
The point that makes the construction obvious is the share split. A company splits its stock, nothing about the business changes, and its influence on the index falls by whatever the split ratio was. Once you have seen that, the number stops being a measurement and starts being a convention.
— Michael Whitman
This page is educational, not financial advice. Test every idea on your own charts before risking money.