The investing term beta coefficient refers to a measure of an asset's volatility relative to a benchmark. Betas are frequently calculated for individual stocks using a benchmark that is representative of the overall stock market.
Beta = Covariance (Asset versus Benchmark) / Variance of the Benchmark
As frequently used in the analysis of stocks:
Stock Beta = Covariance (Stock versus Market Returns) / Variance of the Stock Market
An online beta spreadsheet is available to perform these calculations.
Also referred to as financial elasticity, beta is a measure of statistical variance and uses regression analysis to compare how a single stock moves in relation to the stock market. Typically, an index such as the S&P 500 is used as a benchmark, or proxy, for the entire stock market.
Interpreting beta is fairly straightforward and requires the knowledge of only four rules:
Beta is a measure of risk, and the volatility of an individual stock cannot be mitigated through diversification. It's also an important component of the capital asset pricing model formula. While beta is certainly helpful in understanding the volatility of a stock versus the entire market, it's also a historical measure and should be used with caution. Past performance is never a guarantee of future returns.
The movement of Company A's stock over the last three years was measured against the S&P 500 Index. The beta spreadsheet referenced earlier indicates Company A's beta was 0.72. The interpretation of this value is as follows:
Since the beta is less than 1.0, Company A's stock price swings were less volatile than the entire market. In fact, they move at a rate that was 72% of the market's fluctuations. In addition, since it is a positive value, Company A's stock typically moved in the same direction as the market. That is to say, when the S&P 500 Index increased on a given day, Company A's stock would increase too (in general).