on 9/28/12 at 10:30pm

“So, what did the market do today?”

When people ask that question, most likely they are talking about the Dow Jones Industrial Average. Our minds consciously flip to it anytime there is market news; heck, we even articulate our views on the markets in terms of this index: “Dow’s going to zero. Dow’s going to 20,000. Dow is staying put.”

But how often have you heard: “It doesn’t matter what level it’s at, the Dow is a crappy price-weighted index that nobody should be paying attention to in the first place.”

If you’re a reader of Ken Fisher’s Forbes columns, you have. If you aren’t, you’re probably doing a double take.

How could the Dow be useless, you ask? It’s the single most followed and commented-upon index in the world!

1. It’s only about 29% of total US market cap.

There are only 30 (admittedly very big) stocks in the DJIA, amounting to only about 29% of total US market cap. Thirty. That’s it! At least the S&P500 has around 89% of total US market cap. How do they even choose the stocks included in the index, exactly? Microsoft is in, but no Apple? No Google? Some doctors need to look into this.

2. It’s price weighted. This is beyond stupid. If you answered “yes” to an interviewer’s question on whether a company’s stock price is the best indicator for its inclusion in the world’s most-closely-watched market index, you’d be laughed out of the room. Stock price alone is not an indication of the company’s quality or profitability, but that’s how the Dow is structured. For example, Dow member 3M (MMM) closed today (Sept. 24) at $93.73 per share. Fellow Dow stock Microsoft closed that day at $30.78 per share. That means 3M has three times as much of an impact on the Dow as Microsoft, even though the latter has a much larger market cap. It makes no sense. It would make more sense to weight the index by market size, not stock price, which is arbitrary and distorts the weighting.

3. Stock splits cause all kinds of trouble for the Dow. A stock split doesn’t change anything fundamental about a company. The size of an investors’ position doesn’t change in the event of a stock split, just the number of shares. But because the Dow is price-weighted, stock splits among companies with a higher share price have a bigger effect on the overall index. If a large company like Coca-Cola orders a 2-for-1 split for example, (as it recently did), that’s going to have an impact on the Dow in a negative way even though nothing fundamental has changed about the Coca-Cola company.

And this isn’t just the Dow—the Nikkei has the same wacky price-weighting built into it.

Let’s all do ourselves a favor and use the S&P500 instead!