In key statistics help in finance.yahoo.com you will find the following definition: The Beta used is Beta of Equity. Beta is the monthly price change of a particular company relative to the monthly price change of the S&P500. The time period for Beta is 3 years (36 months) when available. The beta is a measure of the stocks price volatility in relation to the rest of the market. In other words, beta measures the responsiveness of a security to movements in the market portfolio. The market portfolio is assigned a beta of 1. We will use the S&P 500 to represent the market portfolio. There is no single index used to calculate beta, although the S&P 500 is probably the most common proxy for the market as a whole. Beta can be calculated using the standard regression technique. Regression analysis can be done in excel: the Y-variable is the column range for the stock returns and X-variable is the column range for the market returns (S&P 500). Estimate the beta for Hewlett-Packard Company (HPQ) using returns for the most recent 36 months and one of the regression tools available in excel. Discuss the meaning of your results.