Excerpted from Bogle on Mutual Funds by John C. Bogle, page 83

Many fund evaluation services place heavy reliance on Alpha, a term denoting the purported superiority or inferiority of a fund's results. Alpha adjusts the fund's total return for the risk it has assumed, as measured by Beta. Positive Alpha is good, so the argument goes; negative Alpha is bad. But Alphas are volatile and can swiftly move from positive to negative. In my view, Alpha, because of its unpredictable and backward-looking nature is a counterproductive measure. I believe Alpha is a flawed measure of what to expect from a fund and should generally be ignored. On the other hand, ExMarks and Betas of most mature funds with stable investment objectives and policies tend to remain stationary even over decades, making these two concepts far more reliable and useful.