Don't Rely on This Stock Measure

Should you care about a stock's beta? I recently learned that it might be even less of a meaningful sign of a great stock than I'd originally thought.

But let's back up a bit and review what it is. Beta is a measure of a stock's volatility. A beta of 1.0 means that a stock rises and falls in sync with the overall market. A beta greater than 1.0 suggests wider swings, while a beta less than 1.0 indicates a sleepier stock.

Imagine that the stock of Meteorite Insurance (ticker: HEDSUP) has a beta of 1.2. If in the past, the market as a whole advanced 10% in a given period, Meteorite Insurance tended to advance 12%. If the market fell 20%, Meteorite Insurance typically fell around 24%. Conversely, if Acme Explosives Co. (ticker: KABOOM) has a beta of 0.50, it is roughly half as volatile as the market. With a market drop of 8%, we'd expect Acme to slump about 4%. With a market surge of 10%, we'd expect Acme to rise about 5%. Got it?

Beta in real life
Below are the recent betas for some of the Dow's components, just to give you a sense of how some major companies' volatilities compare. I'll also include their star rating from our Motley Fool CAPS community to give you an idea of how bullish on them our thousands of members are:

Data: Motley Fool CAPS, Yahoo! Finance.

As a long-term investor, I haven't paid too much attention to beta. Obviously, if I think the market's going to rise over time, I'd love to have a high-beta stock that outpaces the market's return. But as the examples above show, high-beta stocks during downturns are dangerous to your portfolio.

More importantly, when I buy stock in a company, I usually hope and plan to hang on for a long time. I also expect the stock to rise over that time, ideally substantially. So, to the extent that beta measures volatility, I don't care so much about it. As long as the stock grows over time, I'm not too concerned with how volatile it is as it advances. Many stocks that have proven to be wonderful long-term investments have been very volatile.

What's new?
Here's what I just learned, though: According to an article by researchers Pablo Fernandez and Vicente Bermejo from last month, beta measures vary widely from data provider to data provider. (They mentioned a range of 0.13 to 0.71 for Wal-Mart, for example.) One reason is that the number relies greatly on the time period involved. If one data provider calculates beta based on the past three years and another based on the past five or 10, their results will probably be quite different.

In addition, the researchers concluded that comparing betas of different stocks can be misleading, as even with a given set of data and method of calculation, beta values can differ, making it impossible to be sure that one stock's beta is actually higher than another's.

Lastly, the researchers found that returns of the vast majority of Dow stocks didn't correlate well with their calculated beta values. The researchers found stronger return correlations just by plugging in a fixed beta value of 1. That reduces its usefulness for many investing strategies.

So, even though beta can be interesting as a measure of volatility, don't give the measure more importance than it has. Even if past results suggest a stock will move in a certain direction, always keep in mind that future performance may take a completely different path.

Learn more:

• Can You Stomach Another 40% Drop?
• Early Signs of Winning Stocks
• The 10 Hottest Stocks of 2008