Chapter 11- The CAPM.ppt

1、Chapter 11: The CAPM,Corporate Finance Ross, Westerfield, and Jaffe,Outline,Portfolio theory The CAPM,Expected return with ex ante probabilities,Investing usually needs to deal with uncertain outcomes. That is, the unrealized return can take on any one of a finite number of specific values, say r1,

2、r2, , rS. This randomness can be described in probabilistic terms. That is, for each of these possible outcomes, they are associated with a probability, say p1, p2, , pS. For asset i, its expected return is: E(ri) = p1* r1 + p2* r2 + + pS * rS.,An example, I,Portfolio variability measures with ex an

3、te probabilities,The usual variability measure for a portfolio is variance (and standard deviation); holding other factors constant, the lower the variance (and std.), the better. Variance (and std.) measures the degree of possible deviations from the expected return.,Formulas,Var(r) = p1* (E(r) r1)

4、2 + p2* (E(r) r2)2 + + pS * (E(r) rS)2. Std(r) = Var(r)1/2. Variance and standard deviation are non-negative. Standard deviation has the unit as the original data, whereas variance is just a number (has no unit). For this reason, practitioners prefer using standard deviation.,An example, II,2-asset

5、diversification, I,Suppose that you own $100 worth of IBM shares. You remember someone told you that diversification is beneficial. You are thinking about selling 50% of your IBM shares and diversifying into one of the following two stocks: H1 or H2. H1 and H2 have the same expected rate of return a

6、nd variance (std.).,Portfolio return,Portfolio weight for asset i, wi, is the ratio of market value of i to the market value of the portfolio. The return of a portfolio is the weighted (by portfolio weights) average of returns of individual assets. The expected return of a portfolio is the weighted

7、by portfolio weights) average of expected returns of individual assets.,2-asset diversification, II,2-asset diversification, III,H1 and H2 have the same expected return and variance (std.). Why adding H2 is better than adding H1? The answer is: correlation coefficient. The correlation coefficient b

8、etween IBM and and H2 is lower than that between IBM and H1. That is, with respect to IBMs return behavior, the return behavior of H2 is more unique than that of H1. Return uniqueness is good!,Correlation coefficient,Correlation coefficient measures the mutual dependence of two random returns. Corre

9、lation coefficient ranges from +1 (perfectly positively correlated) to -1 (perfectly negatively correlated). Cov(IBM,H1) = p1* (E(rIBM) rIBM, 1) * (E(rH1) rH1, 1) + p2* (E(rIBM) rIBM, 2) * (E(rH1) rH1, 2) + + pS * (E(rIBM) rIBM, S) * (E(rH1) rH1, S). IBM, H1 = Cov(IBM,H1) / (Std(IBM) * Std(H1) ).,2-

10、asset diversification, IV, = 1, = -1,2-asset diversification,So, these are what we have so far:,The shape of the combinations of 2 assets is like a rubber band. With a low correlation coefficient, you can pull the rubber band further to the left, which is good. Holding other factors constant, the lo

11、wer the correlation coefficient, the better. Again, return uniqueness is healthy!,2-asset formulas,It turns out that there are nice formulas for calculating the expected return and standard deviation of a 2-asset portfolio. Let the portfolio weight of asset 1 be w. The portfolio weight of asset 2 is

12、 thus (1 w). E(r) = w * E(r1) + (1 w) * E(r2). Std(r) = (w2 * Var(r1) + 2* w * (1 w) * cov(1,2) + (1 w)2 * Var(r2)1/2.,Now, let us work on = 0, i.e., Cov=0,N risky assets,The section of the opportunity set above the minimum variance portfolio is the efficient frontier.,return,P,minimum variance port

13、folio,efficient frontier,Individual Assets,N-asset + Rf diversification,Selecting an optimal portfolio from N2 assets,The upper part of the bullet-shape solid line is the efficient frontier (EF): the set of portfolios that have the highest expected return given a particular level of risk (std.). Giv

14、en the EF, selecting an optimal portfolio for an investor who are allowed to invest in a combination of N risky assets is rather straightforward. One way is to ask the investor about the comfortable level of standard deviation (risk tolerance), say 20%. Then, corresponding to that level of std., we

15、find the optimal portfolio on the EF, say the portfolio E shown in the previous figure. CAL (capital allocation line): the set of feasible expected return and standard deviation pairs of all portfolios resulting from combining the risk-free asset and a risky portfolio.,What if one can invest in the

16、risk-free asset?,If we add the risk-free asset to N risky assets, we can enhance the efficient frontier (EF) to the red line shown in the previous figure, i.e., the straight line that passes through the risk-free asset and the tangent point of the efficient frontier (EF). Let us called this straight

17、 line “enhanced efficient frontier” (EEF).,Enhanced efficient frontier (EEF),With the risk-free asset, EEF will be of interest to rational investors who do not like standard deviation and like expected return. Why EEF pass through the tangent point? The reason is that this line has the highest slope

18、 that is, given one unit of std. (variance), the associated expected return is the highest. Why EEF is a straight line? This is because the risk-free asset, by definition, has zero variance (std.) and zero covariance with any risky asset.,Separation, I,When the risk-free asset is available, any eff

19、icient portfolio (any point on the EEF) can be expressed as a combination of the tangent portfolio and the risk-free asset. Implication: in terms of choosing risky investments, there will be no need for anyone to purchase individual stocks separately or to purchase other risky portfolios; the tangen

20、t portfolio is enough.,Separation, II,Once an investor makes the above “investment” decision, i.e., finding the tangent portfolio, the remaining task will be a “financing” decision. That is, including the risk-free asset (either long or short) such that the resulting efficient portfolio meets the in

21、vestors risk tolerance. The “financing” decision is independent (separation) of the “investment” decision.,EEF vs. EF,EEF is almost surely better off than EF, except for the tangent portfolio. In other words, adding the risk-free asset into a risky portfolio is almost surely beneficial. Why? hint: c

22、orrelation coefficient.,When you hold a well-diversified portfolio, I,When you hold a well-diversified portfolio, II,When one holds a well-diversified portfolio, the so called diversifiable (unsystematic, or idiosyncratic) risk disappears. Unsystematic risk: the type of risk that affects a limited n

23、umber of assets. Because unsystematic risk can be easily diversified away by holding a large number of assets, rational investors would not want unsystematic risk in their portfolios. Thus, this type of risk does not require risk premium. Risk premium: the difference between expected return and the

24、risk-free rate.,When you hold a well-diversified portfolio, III,Even when one holds a well-diversified portfolio, the so called un-diversifiable ( or systematic) risk will not be reduced. Systematic risk: the type of (market-wide) risk that affects a large number of assets. Because systematic risk c

25、annot be diversified away, investors need to live with it (monkey on the shoulder) when investing in risky securities. Thus, this type of risk does require risk premium.,Beta as a measure of systematic risk,Systematic risk matters! We use the beta coefficient to measure systematic risk. Beta: a meas

26、ure of the responsiveness of a security to movements in the market. Betai = Cov (i, m) / Var (m).,More about beta,What does beta tell us? A beta 1 implies the asset has more systematic risk than the overall market. The overall market has a beta of 1. The beta of the risk-free asset is 0. Why?,Total

27、risk vs. systematic risk,Consider the following information:Standard Deviation Beta Security A 15% 1.50 Security B 30% 0.50 Which security has more total risk? Which security has more systematic risk? Which security should have a higher expected return?,Risk, again,For a portfolio, we care about var

28、iance and standard deviation. But this risk concept at portfolio level does not automatically carry forward to individual security level. For a security, we care about beta. The reason is that variance and standard deviation do not add up.,Beta and risk premium,So far, we know that beta is a measure

29、 of systematic risk, and bearing systematic risk requires compensation in the form of extra return (expected return). Thus, the higher the beta, the greater the risk premium. This relationship is depicted in the following figure.,Beta and expected return, rf = 6%,Reward-to-risk ratio,The reward-to-r

30、isk ratio is the slope of the red line illustrated in the previous figure: slope = (E(ri) rf) / (i 0). The red line is called “security market line (SML).” What if an asset, j, has a higher reward-to-risk ratio than the red line? j is an bargain and the market is not in equilibrium. Investors (and t

31、heir demand) will bid up js price, drive down its expected return, and make its reward-to-risk ratio equal to that of the red line.,Equilibrium argument,The previous equilibrium argument ensures that all assets and portfolios will have the same reward-to-risk ratio and they all must equal the reward

32、to-risk ratio of the overall market, i.e., the market portfolio. That is, (E(ri) rf) / i = (E(rm) rf) / m. Recall that m = 1. Then, we have the CAPM: E(ri) = rf + i (E(rm) rf).,The CAPM,The CAPM says that in equilibrium, all securities and portfolios should fall on the security market line, i.e., t

33、he red line. The CAPM says that the higher the beta, the higher the expected return. One can think of the expected return being the required return given the assets beta. For capital budgeting, the required return is frequently used as the cost of equity; the return required by equity (stock market)

34、 investors.,An example,Suppose that the beta estimate for MMM is 1.5 (). The current T-bill rate is 5%. We know that historical risk premium for S&P 500 Index is about 8.5%. What is the cost of equity for MMM?E(ri) = rf + i (E(rm) rf) = 5% + 1.5 8.5% = 17.75%.,All-equity firms,One uses the cost of e

35、quity as the discount rate if (1) the firm uses no debt, or (2) the cash flows being discounted are the kind of cash flows available to equityholders. If the previous firm (i.e., MMM) uses no debt, the required return (the discount rate) for evaluating a new project (in terms of computing the NPV) i

36、s the cost of equity, 17.75%.,This is the way practitioners estimate ,Characteristic line,The previous regression line is called the characteristic line. The slope of the characteristic line is an estimate of . The intercept term estimate is called “alpha.”,Is the CAPM a good model?,It is a beautifu

37、l model. It does not have desirable empirical properties. In recent years, more and more people would like to see a better way of estimating the cost of equity. Possible directions: multi-factor models? real-option-based models?,Assignment,Suppose that mutual fund A has an expected return of 10% and

38、 a standard deviation of 15%. Mutual fund B has an expected return of 15% and a standard deviation of 30%. The correlation coefficient between A and B is +0.3. (1) Please plot the feasible set or the opportunity set, i.e., attainable portfolios, by alternating the mix between the two funds. (2) What

39、 are the expected return and standard deviation for a portfolio comprised of 30% fund A and 70% fund B? (3) Suppose that the risk-free asset has an expected return of 5%. Using only fund B and the risk-free asset, plot the feasible set. Due in a week.,End-of-chapter,Concept questions: 1-10. Questions and problems: 1-32.,