How the Dow's Price-Weighted Scoring Played Out Today

The Dow Jones Industrial Average (DJINDICES: ^DJI  ) scores are calculated based on share prices, not percentage moves or market caps. It's a somewhat controversial choice that makes a big difference to the way share price swings affect the index. Here's how this choice played out on Wednesday. (All stock prices are current as of 2 p.m. EDT today.)

How the Dow Is Weighted | Infographics.

That's the basics. Click through the three views of this data and you'll notice that nothing much changes. By dint of having the largest share price by a wide margin, IBM (NYSE: IBM  ) dominates the Dow like nobody else. Big Blue accounts for more than 10% of the total Dow score. A 1% change in IBM's share price will add or remove about 16 Dow points.

The second-most impactful Dow stock, oil giant Chevron, would have to score a 1.7% gain to move the Dow's needle that far.

With a single-digit share price, aluminum producer Alcoa (NYSE: AA  ) represents just 63 Dow points overall. The stock needs a 25% spike to match the 16-point impact of a 1% move in IBM's prices. You can explore the spread between percentage moves and Dow point impacts in the next chart:

What the Weightings Mean | Create infographics.

So, for example, Hewlett-Packard notched the largest percentage gain on the Dow this afternoon -- a respectable 0.7%. That was worth just 1.3 Dow points due to HP's low share price. IBM lost 0.6%, which translates into 9.1 Dow points. That's the second-largest move (up or down) for the day, though it's only the 11th-largest percentage loss.

Biggest loser Verizon (NYSE: VZ  ) saw smaller wireless rivals moving closer to a market-changing consolidation and lost 1.5%. That's tied for the third-widest Dow point loss, at 5.8%, leapfrogged by Big Blue and Boeing (NYSE: BA  ) .

Boeing scored the rare combination of a large percentage move and triple-digit share price. Archrival Airbus just landed two massive orders for its brand new A350 jumbo jets, robbing Boeing of potential Dreamliner sales. That's good for 10.4 points of Dow damage.

This is why the Dow doesn't always reflect the health of the overall market. A significant move by IBM or any of its triple-digit share-price pals can -- and often does -- steamroll over the swings of less pricey Dow peers. To underscore the skewed nature of the oft-referenced index, let me point out that IBM holds just 5% of the Dow's total market-cap value, 3.4% of the Dow's total trailing revenue, and 5.6% of total earnings.

Big Blue may be a giant, but with a 10% index impact, it's clearly over-represented among Dow stocks.

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