Capital Asset Pricing Model and Systematic Risk

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THE CAPITAL ASSET PRICING MODEL RELEVANT TO ACCA QUALIFICATION PAPER F9 Section F of the Study Guide for Paper F9 contains several references to the capital asset pricing model (CAPM). This article is the last in a series of three, and looks at the theory, advantages, and disadvantages of the CAPM. The first article, published in the January 2008 issue of student accountant introduced the CAPM and its components, showed how the model can be used to estimate the cost of equity, and introduced the asset beta formula. The second article, published in the April 2008 issue, looked at applying the CAPM to calculate a project-specific discount rate to use in investment appraisal.

CAPM FORMULA The linear relationship between the return required on an investment (whether in stock market securities or in business operations) and its systematic risk is represented by the CAPM formula, which is given in the Paper F9 Formulae Sheet: E(ri) = Rf + βi(E(rm) - Rf) E(ri) = return required on financial asset i Rf = risk-free rate of return βi = beta value for financial asset i E(rm) = average return on the capital market The CAPM is an important area of financial management. In fact, it has even been suggested that finance only became ‘a fully-fledged, scientific discipline’ when William Sharpe published his derivation of the CAPM in 19861. CAPM ASSUMPTIONS The CAPM is often criticised as being unrealistic because of the assumptions on which it is based, so it is important to be aware of these assumptions and the reasons why they are criticised. The assumptions are as follows2:

Investors hold diversified portfolios This assumption means that investors will only require a return for the systematic risk of their portfolios, since unsystematic risk has been removed and can be ignored. Single-period transaction horizon A standardised holding period is assumed by the CAPM in order to make comparable the returns on different securities. A return over six months, for example, cannot be compared to a return over 12 months. A holding period of one year is usually used. Investors can borrow and lend at the risk-free rate of return This is an assumption made by portfolio theory, from which the CAPM was developed, and provides a minimum level of return required by investors. The risk-free rate of return corresponds to the intersection of the security market line (SML) and the y-axis (see Figure 1). The SML is a graphical representation of the CAPM formula. Perfect capital market This assumption means that all securities are valued correctly and that their returns will plot on to the SML. A perfect capital market requires the

following: that there are no taxes or transaction costs; that perfect information is freely available to all investors who, as a result, have the same expectations; that all investors are risk averse, rational and desire to maximise their own utility; and that there are a large number of buyers and sellers in the market. FIGURE 1: THE SECURITY MARKET LINE Return E(ri) Rm Rf SML



While the assumptions made by the CAPM allow it to focus on the relationship between return and systematic risk, the idealised world created by the assumptions is not the same as the real world in which investment decisions are made by companies and individuals.

technical page 51

linked performance objectives performaNce obJectives 15 aNd 16 are reLevaNt to paper f9

For example, real-world capital markets are clearly not perfect. Even though it can be argued that well-developed stock markets do, in practice, exhibit a high degree of efficiency, there is scope for stock market securities to be priced incorrectly and, as a result, for their returns not to plot on to the SML. The assumption of a single-period transaction horizon appears reasonable from a real-world perspective, because even though many investors hold securities for much longer than one year, returns on securities are...
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