Excerpted from Bogle on Mutual Funds by John C. Bogle, page 177

There is one final problem in selecting a winning manager. According to Richard A. Brealey, one of the most respected pioneers of capital market theory, "... you probably need at least 25 years of fund performance to distinguish at the 95% significance level whether a manager has above-average competence." Another commentator accepted the 15-year time frame, "but only if the pension executive is using the perfect (italics supplied) benchmark for that manager. Using a less than perfect benchmark may increase the observation time to 80 years." Now that is a long time to wait for confirmation. The logic of adopting an equity investment strategy focused at least in part on indexing seems almost overpowering.