How to calculate Jensen's alpha
Worked example
- Portfolio return 15%, risk-free rate 2%, market return 10%, β = 1.2.
- Expected return = 2% + 1.2 × (10% − 2%) = 11.6%.
- α = 15% − 11.6% = +3.4%.
How to interpret Jensen's alpha
Positive alpha means the portfolio earned more than its market exposure justified; negative alpha means it earned less. A high-beta portfolio that beat the market may still have negative alpha if the extra return was only compensation for the extra risk. Estimate beta with the beta calculator.
Limitations and common mistakes
- Use returns, risk-free rate and market return over the same period.
- Alpha over a single year is mostly noise; look for persistence over many periods.