The CAPM is also often used to measure the performance of mutual funds and
other managed portfolios. The approach, dating to Jensen (1968), is to estimate
the CAPM time-series regression for a portfolio and use the intercept Uensen's
alpha) to measure abnormal performance. The problem is that, because of the
empirical failings of the CAPM, even passively managed stock portfolios produce
abnormal returns if their investment strategies involve tilts toward CAPM problems
(Elton,Gruber, Das and Hlavka, 1993).For example, funds that concentrate on low
beta stocks, small stocks or value stocks will tend to produce positive abnormal
returns relative to the predictions of the Sharpe-Lintner CAPM, even when the
fund managers have no special talent for picking u'inners.
The CAPM, like Markowitz's (1952, 1959) portfolio model on which it is built,
is nevertheless a theoretical tour de force. We continue to teach the CAPM as an
introduction to the fundamental concepts of portfolio theory and asset pricing, to
be built on by more complicated models like Merton's (1973) ICAF'M. But we also
warn students that despite its seductive simplicity, the CAPM's empirical problems
probably invalidate its use in applications.