In practice, beta is calculated using historical returns for both the asset and the market, with the market portfolio being represented by a broad index such as the S&P 500 or the Russell 2000. This type of data is widely available from financial databases and can be downloaded into software packages like Excel or SPSS for easy manipulation.
To determine the beta of a portfolio, we simply average the individual securities’ betas, weighted by the market capitalization of each security.
The next section describes how such a measure of risk can be used in a model to describe the relationship between systematic risk and expected return.