Portfolio alpha is obtained by measuring the difference between the return of the portfolio and that of a benchmark portfolio. This measure appears to be the only reliable performance measure to evaluate active management. In fact, we have to distinguish between normal returns, provided by the fair reward for portfolio exposure to different risks, and obtained through passive management, from abnormal performance (or outperformance) due to the manager's skill (or luck), whether through market timing, stock picking, or good fortune. The first component is related to allocation and style investment choices, which may not be under the sole control of the manager, and depends on the economic context, while the second component is an evaluation of the success of the manager's decisions. Only the latter, measured by alpha, allows the evaluation of the manager's true performance (but then, only if you assume that any outperformance is due to skill and not luck).