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 in the way share-price swings affect the index. Here's how this choice has played out today (numbers are current as of 1 p.m. EDT).

How the Dow Is Weighted | Infographics.

Those are the basics. Click through the three views of this data and you'll notice that nothing much changes. IBM holds the largest share price on the index by a wide margin -- 67% higher than the nearest comparison -- and so it 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-weightiest Dow stock, oil giant Chevron, would have to score a 1.7% gain to move the Dow's needle that far.

And with its single-digit share price, aluminum producer Alcoa represents just 62 Dow points overall. The stock would need a 26% 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 the market took a nosedive on Thursday. Housing trends aren't what they should have been, Ben Bernanke signaled a probable end to the long-running federal stimulus program, and the world is plainly coming to an end. These factors combined for a 1.3% drop in the Dow at midday.

It's a large move that kind of hurts to look at, but I hope you heard the tongue in my cheek as I talked about the end of the world. The second coming of 1929's Black Tuesday this is not, nor is it a repeat of the Lehman Brothers panic.

Let me remind you that the Dow has dropped more than 1.3% overnight 10 times in the last year, and the sky never fell. In fact, the Dow jumped 1.3% or more 12 times in the same period and is up 16.3% all things considered.

These hiccups happen. And the Dow isn't always the best barometer of overall market health, either.

For example, Walt Disney (NYSE:DIS) suffered a 2.6% swoon -- the second-worst overnight drop on the Dow today -- thanks to general market panic plus a downgrade from influential analyst firm Goldman Sachs. But Disney's shares trade at just $63 apiece, so the Mouse's move only shaved 12 points off the Dow. Big Blue's much narrower 1.4% loss translated into 21 Dow points lost, presumed dead.

For an even starker example, look at chip giant Intel (NASDAQ:INTC). The Dow's biggest day-to-day drop, just slightly higher than Disney's 2.6%, translates to no more than five Dow points when you do the math. Those are the breaks when your share price hovers in the $20 to $30 range, as Intel's does.

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 10.3% index impact, it's clearly over-represented among Dow stocks.

Fool contributor Anders Bylund holds no position in any company mentioned. Check out Anders' bio and holdings or follow him on Twitter and Google+.

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