In the context of CAPM, calculating alpha requires the following inputs:
- the realized return (on the portfolio),
- the market return,
- the risk-free rate of return, and
- the beta of the portfolio.
Jensen's alpha = Portfolio Return − [Risk Free Rate + Portfolio Beta * (Market Return − Risk Free Rate)]
alpha_J = R_i - [R_f + eta_{iM} cdot (R_M - R_f)]
An additional way of understanding the definition can be obtained by rewriting it as:
alpha_J = (R_i - R_f) - eta_{iM} cdot (R_M - R_f)
If we define the excess return of the fund (market) over the risk free return as Delta_R equiv (R_i - R_f) and Delta_M equiv (R_M - R_f) then Jensen's alpha can be expressed as:
alpha_J = Delta_R - eta_{iM} Delta_M