Excerpted from Bogle on Mutual Funds by John C. Bogle, pages 78-79

Find out whether a portfolio manager has run the fund for a few months, a few years, or a few decades, and give this information whatever weight you deem appropriate. That said, many fine equity funds are run by teams of managers, with the advisory firm as an organization putting its corporate stamp on the funds' strategies and their implementation. Because of this diversity, such funds tend to be classic mainstream funds, and the tenure of a single manager should be relatively inconsequential.

Even when funds have individual portfolio managers, performance in a particular period can be due to much more - or less - than the manager's skill. For instance, a manager may be less important than the research and analytical support he receives. Or a manager may grow, or shrink, in capability. It is not unknown for a new manager to do better than a successful predecessor. A market environment in which a manager has been able to shine brightly may be replaced by a very different environment that does not favor the manager's investment style. Finally, good luck (always a factor in shaping fund returns) may turn to bad, and vice versa. Tenure is but one factor for you to consider. When managers change, a wait-and-see policy is usually appropriate.