On a morning when Nasdaq futures are sliding and Dow futures are rising, beginners often ask the obvious question: how can two measures of the US stock market point in opposite directions? A big part of the answer is how each index is built. The Dow Jones Industrial Average is a price-weighted index, and that one design choice changes which companies matter most.
What a price-weighted index is
A price-weighted index gives each company influence according to its share price, not according to how big the company is. A stock trading at $600 moves the index far more than one trading at $60, even if the cheaper share belongs to a much larger business. The index is calculated by adding up the share prices of its members and dividing by a number called the divisor, which is adjusted over time so that events like stock splits do not cause artificial jumps.
The Dow, with its 30 large US companies, is the best-known example. Japan’s Nikkei 225 is another. Most modern indices, including the S&P 500, the FTSE 100 and the Nasdaq 100, use market capitalisation weighting instead, where the biggest companies by total value carry the most weight. We explain that approach in our guide to market-cap weighting versus equal weight.
Why it matters
Because the Dow is price-weighted and holds only 30 companies, it can behave very differently from the broader market. Some of the largest technology companies in the world carry a far smaller say in the Dow than they do in the S&P 500 or Nasdaq, and some have relatively modest share prices after splits. Meanwhile, a high-priced industrial, insurer or bank can swing the Dow by dozens of points on its own.
That is why, on a day when a technology giant disappoints after results, you can see Nasdaq futures fall sharply while Dow futures hold up or even rise. The Dow is not saying the economy is fine and the Nasdaq is saying it is not. They are simply weighting different businesses in different ways.
A simple example
Imagine an index of three shares priced at $500, $100 and $20. If the $500 share rises 2 per cent, it adds $10 to the total. If the $20 share rises 2 per cent, it adds just 40 cents. The same percentage move has twenty-five times the impact, purely because of the starting share price. In a market-cap index, what would matter instead is how much each company is worth in total.
Stock splits make this stranger still. If a company splits its shares two-for-one, its price halves and so does its influence on a price-weighted index, even though nothing about the business has changed.
How traders use this
Traders mostly use the distinction to read the market correctly. When the Dow and the Nasdaq diverge, it usually tells you which sectors are driving the day. A strong Dow alongside a weak Nasdaq often points to money rotating out of technology into industrials, financials or healthcare. A weak Dow on a strong Nasdaq may point the other way.
It also helps to check which individual stocks are behind a big Dow move. A 300-point swing can sound dramatic, but if most of it comes from one high-priced member reacting to its own results, it says little about the wider economy. With the Dow above 50,000, it is also worth thinking in percentages rather than points.
Common mistakes
The most common mistake is treating the Dow as the whole US market. It is a useful, historic index, but it is narrow and its weighting is quirky. Another mistake is reading a points move as big news without checking the percentage. A third is assuming that because the Dow is up, technology shares in your portfolio will be too.
The bottom line
A price-weighted index lets share prices, not company size, decide influence. That makes the Dow a different animal from the S&P 500 and the Nasdaq, and understanding why they diverge is one of the quickest ways to read what is really moving the market on any given day. If you want to see how well you read days like these, the free Samuel & Co Trading Trader Assessment is a good next step.
