CAPM can be extended in a number of areas and provide additional applications beyond the estimation of security returns. A key area is in the evaluation of performance where a number of commonly used metrics are used.
There are various performance metrics that can be computed that are extensions of CAPM and allow for the assessment of portfolio performance and evaluation. Active managers are expected to perform better than passive managers or to at least cover the costs of active management. There are four ratios commonly used in performance evaluation. All measures assume that the benchmark market portfolio is the correct portfolio and if not, may cause the results to be misleading. The benchmark should be appropriate for the portfolio being measured and should exhibit similar characteristics. We see these performance measures in detail in the next LOS.
The CAPM assumes investors have homogeneous expectations, are rational and risk-averse and thus assign the same value to all assets to create the same risky market portfolio. If investors are heterogeneous, their different beliefs could result in a valuation or price for a security different from the CAPM-calculated price. The CAPM-calculated price is the current market price because it reflects the beliefs of all other investors in the market. If the investor’s estimated price is higher than the current market price, this could then provide an indication to buy the asset as it is considered undervalued by the market.
A Jensen’s alpha for individual securities can also be computed with positive values indicating the security is likely to outperform the market on a risk-adjusted basis.
Similar information can be represented graphically by a Security Market Line (SML). The expected return and beta for a security can be assessed against the SML with those securities that are undervalued relative to market consensus appearing above the SML. Securities overvalued relative to market consensus will appear below the SML.
CAPM suggests that investors should hold the market portfolio and a risk-free asset. The true market portfolio consists of a large number of securities and it may not be practical for an investor to own them all. Much of the non-systematic risk can be diversified away by holding 30 or more individual securities, however, these securities should be randomly selected from multiple asset classes. An index may serve as the best method of creating diversification.
It is important to note that only non-systematic risk can be eliminated through the addition of different securities into the portfolio. Systematic risk – the risk inherent to the entire market – cannot be diversified away.
Securities not included within the index can be evaluated relative to the index to determine their suitability for portfolio inclusion. The alpha and beta of the security can be estimated relative to the index and those with a positive alpha should be included. The same exercise can be conducted for securities within the index – those with negative alphas relative to the index should be excluded, or sold short.
To determine the weight of each security within the portfolio, those securities with higher alpha should be given more weight with this weight proportional to the alpha divided by the non-systematic variance (risk) of the security.
Limitations of CAPM
The CAPM is subject to theoretical and practical implications. From a theoretical perspective, it is both a single-factor and single-period model. There may be other factors over multi-time periods that would be more appropriate in modeling expected returns. Practically, the following are the limitations:
- Market portfolio: the true market portfolio includes all assets – financial and non-financial which may not be investable or tradeable.
- Proxy for market portfolio: typically, a proxy for the market portfolio is used, but different analysts tend to use different proxies.
- Estimation of beta risk: a long history is required to estimate beta, however, the history may not be an accurate representation of the future beta and different historical time periods (3 years versus 5 years) and different data frequency (daily versus monthly) are likely to produce different betas.
- Poor predictor of returns: the empirical support for CAPM is weak – the model is not good at predicting future returns which in turn indicates that asset returns cannot be determined solely by systematic risk.
- Homogeneity in investor expectations: in reality, investors are unlikely to have homogeneous expectations – there will be many optimal risky portfolios and numerous Security Market Lines (SMLs).
An overvalued security would most likely plot:
A. Below the Security Market Line (SML)
B. On the Security Market Line (SML)
C. Above the Security Market Line (SML)
The correct answer is A.
Securities overvalued relative to market consensus will appear below the SML while securities undervalued relative to market consensus will appear above the SML. Securities correctly priced will appear directly on the SML.
Reading 53 LOS 53h:
Describe and demonstrate applications of the CAPM and the SML