Look at the companies listed in Table 8.2. Calculate monthly rates of return for two successive five-year periods. Calculate betas for each subperiod using the Excel SLOPE function. How stable was each companys beta? Suppose that you had used these betas to estimate expected rates of return from the CAPM. Would your estimates have changed significantly from period to period?

Stock Bcta (B) United States Steel 3.01 Amazon 1.47 Southwest Airlines 1.35 The Travelers Companies 1.26 Tesla 0.94 ExxonMobil 0.82 Johnson & Johnson 0.81 Coca-Cola 0.70 Consolidated Edison 0.11 Newmont 0.10 Stock Bcta (B) United States Steel 3.01 Amazon 1.47 Southwest Airlines 1.35 The Travelers Companies 1.26 Tesla 0.94 ExxonMobil 0.82 Johnson & Johnson 0.81 Coca-Cola 0.70 Consolidated Edison 0.11 Newmont 0.10