Alpha

Alpha is the excess return a portfolio or strategy produces beyond what its exposure to a benchmark or set of risk factors would predict. An alpha of zero means performance is fully explained by that exposure; a positive alpha implies genuine outperformance, or at least unexplained return that may or may not persist.

Alpha is typically estimated by regressing a portfolio's returns against a benchmark's returns: Rp = alpha + beta * Rb + error, where beta captures sensitivity to the benchmark and alpha is the regression intercept. In multi-factor investing models, alpha is what remains after controlling for several risk factors rather than one market benchmark, so the same track record can show a large single-factor alpha and a much smaller multi-factor alpha.

Alpha is the central justification for active management: managers charge fees on the premise of generating it, and it is the standard yardstick used in fundamental analysis and elsewhere for comparing skill across funds. The common misreading is treating a positive historical alpha as proof of skill rather than possibly luck, an omitted risk factor, or a period-specific effect — alpha estimated over short windows or bull markets is notoriously unstable and can shrink or vanish once the sample period or factor model changes.