Alpha
The return earned beyond what the portfolio's market risk explains: what is left after subtracting beta multiplied by the market's own move.
Formula
\alpha = r_p - \left[r_f + \beta\left(r_m - r_f\right)\right]Computed over matching periods: the portfolio return less the return the market would have delivered at that beta. The result is a fraction for the period, not annualised.
How to read the number
The number depends on the chosen benchmark: against one index the portfolio shows a surplus, against another a shortfall, while the trades were identical.
When the metric lies
Over a short window it cannot be told apart from luck: at ordinary market dispersion, separating skill from chance needs years of history rather than quarters.
Also known as: jensen alpha