Most of the time, you'll hear investors and financial advisors mention "beta" when they're talking about asset allocation and diversification. This is because beta represents a concept called correlation; how closely a stock price will mirror the performance of an index or "the market".
Here's a brief overview of the concept, via slideshare:
In theory, a group of stocks with a beta equal to zero will rise and fall in price independently of the general market. For you, this means that your investing returns will remain steady and consistent.
Keep in mind that beta comes from a model. And as with any model, it is an approximation and does not hold true in every single situation. Think of it as a guideline, rather than a rule or law.
Since the markets are often thought of as an economic system, beta is also called a measure of "market" or "systematic" risk. Basically, this refers to the risk caused by price movements of the general market.
This is not to be confused with "systemic" risk, which is the possibility that an event at the company level could trigger severe instability or collapse an entire industry or economy...this Lehman brothers and the 2008 financial crisis.
The Beta Calculation
For many, this is a chance to plug-in the 6%-8% long term return of the "market". Keep in mind that if you choose to use a "long-term" percentage, the Beta you calculate will also be for the same "long-term".
The rub is that the longer the outlook, the more uncertainty exists. And higher uncertainty means that your calculation for Beta is less likely to be correct.
Historically, U.S. Treasuries are the closest thing to a risk-free investment, because they are guaranteed by the United States government.
So the expected return of U.S. Treasuries is used as a substitute for risk-free return, because it is as close as you can get.
To keep things simple, I assumed an expected market return for the S&P500 of 8% and a risk-free return of 2%. And with a little research, I found that the VFINX mutual fund has an expense ratio of 0.18%. For more information on the importance of the expense ratio on your investments, click here.
Now we can plug our numbers into the beta equation:
Beta = (7.82%-2.00%)/(8.00%-2.00%) = 0.97
This means that for every 1% move in the S&P500, VFINX can be expected to move 0.97%.
So, if the S&P500 goes up 10% next year, you can expect VFINX to rise 9.7%. Or, if the S&P falls 10% next year, you can expect VFINX to fall 10.3% (because you still have to pay the expense ratio regardless of performance!).
Safe Investing Tip:
If the expected return of an index fund isn't REALLY REALLY REALLY close to the expected return for the index it follows, it is NOT a good index fund!