The Security Market Line (SML)
Required Rate SML: r i = rRF + (rM ā“ rRF) bi
of Return (%) = 6% + (11% ā“ 6%) bi
= 6% + (5%) bi
rHigh = 16
Market Risk Risky Stockā™s
rM = rA = 11
Premium: 5%. Risk Premium: 10%
Applies Also to
an Average Stock,
rLow = 8.5 and Is the Slope
Coefficient in the
0 0.5 1.0 1.5 2.0 Risk, b i
28-18 Basic Financial Tools: A Review
combinations of the market portfolio and the risk-free asset. It shows the
best available set of portfolios, based on risk and return, available to
investors. The SML, on the other hand, shows the relationship between the
required rate of return on individual stocks and their market risk as mea-
sured by beta.14
The Characteristic Line: Calculating Betas
Before we can use the SML to estimate a stockā™s required rate of return, we
need to estimate the stockā™s beta coefļ¬cient. Recall that beta is a measure of
how the stock tends to move with the market. Therefore, we can use the his-
torical relationship between the stockā™s return and the marketā™s return to cal-
culate beta. First, note the following deļ¬nitions:
rJ historical (realized) rate of return on Stock J. (Recall that rJ and rJ
are deļ¬ned as Stock Jā™s expected and required returns, respectively.)
rM historical (realized) rate of return on the market.
aJ vertical axis intercept term for Stock J.
bJ slope, or beta coefļ¬cient, for Stock J.
eJ random error, reļ¬‚ecting the difference between the actual return on
Stock J in a given year and the return as predicted by the regression
line. This error arises because of unique conditions that affect Stock
J but not most other stocks during a particular year.
The points on Figure 28-4 show the historical returns for Stock J plotted
against historical market returns. The returns themselves are shown in the
bottom half of the ļ¬gure. The slope of the regression line that best ļ¬ts these
points measures the overall sensitivity of Stock Jā™s return to the market
return, and it is the beta estimate for Stock J. The equation for the regres-
sion line can be obtained by ordinary least squares analysis, using either a
calculator with statistical functions or a computer with a regression software
package such as a spreadsheetā™s regression function. In his 1964 article
which ļ¬rst described the CAPM, Sharpe called this regression line the stockā™s
characteristic line. Thus, a stockā™s beta is the slope of its characteristic line.
Figure 28-4 shows that the regression equation for Stock J is as follows:
rJ 8.9 1.6 r M eJ. (28-17)
This equation gives the predicted future return on Stock J, given the mar-
ketā™s performance in a future year.15 So, if the marketā™s return happens to be
20 percent next year, the regression equation predicts that Stock Jā™s return
will be 8.9 1.6(20) 23.1%.
We can also use Equation 28-16 to determine the required rate of return
on Stock J, given the required rate of return on the market and the risk-free
rate as shown on the Security Market Line:
rJ rRF 1.6(rM rRF).
Required rates of return are equal to expected rates of return as seen by the marginal investor if markets are efļ¬cient
and in equilibrium. However, investors may disagree about the investment potential and risk of assets, in which case the
required rate of return may differ from the expected rate of return as seen by an individual investor.
This assumes no change in either the risk-free rate or the market risk premium.
Risk and Return
Calculating Beta Coefficients
on Stock J, r J (%)
40 Year 5
30 _ _
r J = a J + b J r M + eJ
= ā“ 8.9 + 1.6r M + e J
ā“10 0 10 20 30 Realized Returns
on the Market, r M (%)
a J = Intercept = ā“ 8.9% _
ā rJ = 8.9% + 7.1% = 16%
_ Rise 16
ā r M = 10% = _J =
bJ = = 1.6
STOCK J ( ā“J ) MARKET ( ā“M )
YEAR r r
1 38.6% 23.8%
2 (24.7) (7.2)
3 12.3 6.6
4 8.2 20.5
5 40.1 30.6
Average r 14.9% 14.9%
Ļr 26.5% 15.1%
Thus, if the risk-free rate is 8 percent and the required rate of return on the
market is 13 percent, then the required rate of return on Stock J is 8%
1.6(13% 8%) 16%.
Market versus Diversifiable Risk
Equation 28-17 can also be used to show how total, diversiļ¬able, and
market risk are related. The total risk for Stock J, 2, can be broken down
into market risk and diversiļ¬able risk:
Total risk Variance Market risk Diversifiable risk. (28-18)
b2 2 2
28-20 Basic Financial Tools: A Review
Here J is Stock Jā™s variance (or total risk), bJ is the stockā™s beta coefļ¬cient,
M is the variance of the market, and eJ is the variance of Stock Jā™s regres-
sion error term. If all of the points in Figure 28-4 plotted exactly on the
regression line, then J would have zero diversiļ¬able risk and the variance of
the error term, eJ, would be zero. However, in our example all of the points
do not plot exactly on the regression line, and the decomposition of total
risk for J is thus
Total risk Market risk Diversifiable risk
terms, the market risk is 20.0584 24.2%, and the diversiļ¬able risk is
0.0702 0.0584 eJ.
Solving for eJ gives diversiļ¬able risk of eJ 0.0118. In standard deviation
The beta of a portfolio can be calculated as the weighted average of the betas
of the individual assets in the portfolio. This is in sharp contrast to the dif-
ļ¬cult calculation required in Equation 28-13 for ļ¬nding the standard devia-
tion of a portfolio. To illustrate the calculation, suppose an analyst has
determined the following information for a four-stock portfolio:
Stock Weight Beta Product
(1) (2) (3) (4) (2) (3)
I 0.40 0.6 0.24
J 0.20 1.0 0.20
K 0.30 1.3 0.39
L 0.10 2.1 0.21
1.00 Portfolio beta: 1.04
The beta is calculated as the sum of the product terms in the table. We could
also calculate it using this equation:
bp 0.4(0.6) 0.2(1.0) 0.3(1.3) 0.1(2.1) 1.04.
Here the ļ¬rst value in each term is the stockā™s weight in the portfolio, the
second term is the stockā™s beta, and the weighted average is the portfolioā™s
beta, bp 1.04.
Now assume that the risk-free rate is 8 percent, and the required rate of
return on the market portfolio is 12 percent. The portfolioā™s required return
can be found using Equation 28-16:
rp 8% 1.04(12% 8%) 12.16%.
How are the expected return and standard deviation calculated from a proba-
When is the coefficient of variation a useful measure of risk?
What is the difference between diversifiable risk and market risk?
Explain the following statement: āAn asset held as part of a portfolio is gener-
ally less risky than the same asset held in isolation.ā
What does it mean to say that beta is the theoretically correct measure of a
What is the difference between the CML and the SML?
How would you calculate a beta?
What is optimal about an āoptimal portfolioā?
The techniques used earlier in this chapter to value bonds can also be used,
with a few modiļ¬cations, to value stocks. First, note that the cash ļ¬‚ows
from stocks that companies provide to investors are dividends rather than
coupon payments, and there is no maturity date on stocks. Moreover, divi-
dend payments are not contractual, and they typically are expected to grow
over time, not to remain constant as bond interest payments do. Here is the
general equation used to value common stocks:
D1 D2 Dn
Price PV . (28-19)