Transcript Capital Asset Pricing Model

Capital Asset Pricing Model
CAPM I: The Theory
Introduction
• Asset Pricing – how assets are priced?
• Equilibrium concept
• Portfolio Theory – ANY individual
investor’s optimal selection of portfolio
(partial equilibrium)
• CAPM – equilibrium of ALL individual
investors (and asset suppliers)
(general equilibrium)
Intuition
• Risky Asset i:
• Its price is such that:
E(Returni) = Risk-free rate of return + Risk premium specific to Asset i
= Rf + (Market price of risk)x(quantity of risk of asset i)
CAPM tells us 1) The price of risk
2) The risk of Asset i?
An example to motivate
Expected Return
Standard Deviation
Asset I
10.9%
4.45%
Asset j
5.4%
7.25%
E(return) = Risk-free rate of return + Risk premium specific to asset i
= Rf + (Market price of risk)x(quantity of risk of asset i)
Question: According to the above equation, given that asset j has higher risk
relative to asset i, why wouldn’t asset j has higher expected return as well?
Possible Answers:
(1) The equation, as intuitive as it is, is wrong.
(2) The equation is right, but the market prices of
risk are different for different assets.
(3) The equation is right, but the quantity of risk of
any risky asset is not equal to the standard
deviation of its return.
Answers from CAPM
E(return) = Risk-free rate of return + Risk premium specific to asset i
= Rf + (Market price of risk)x(quantity of risk of asset i)
• The intuitive equation is right.
• The market price of risk in equilibrium
should be the same across ALL
marketable assets
• In the equation, the quantity of risk of any
asset, however, is only PART of the total
risk (s.d) of the asset.
CAPM’s Answers
• Specifically:
Total risk = systematic risk + unsystematic risk
CAPM says:
(1)Unsystematic risk can be costlessly diversified away.
“No free lunch” implies the market will NOT reward you
for bearing unsystematic risk if there is NO cost of
eliminating it through well diversification.
(2)Systematic risk cannot be diversified away without cost.
In other words, investors need to be compensated by a
certain risk premium for bearing systematic risk.
CAPM results
E(return) = Risk-free rate of return + Risk premium specific to asset i
= Rf + (Market price of risk)x(quantity of risk of asset i)
Precisely:
[1] Expected Return on asset i = E(Ri)
[2] Equilibrium Risk-free rate of return = Rf
[3] Quantity of risk of asset i = COV(Ri, RM)/Var(RM)
[4] Market Price of risk = [E(RM)-Rf]
The following equation, a.k.a., the Capital Asset Pricing Model:
E(Ri) = Rf + [E(RM)-Rf] x [COV(Ri, RM)/Var(RM)]
Where [COV(Ri, RM)/Var(RM)] is referred to as the BETA of asset i
Or
E(Ri) = Rf + [E(RM)-Rf] x βi
Pictorial Result of CAPM
E(Ri)
Security
Market
Line
E(RM)
Rf
slope = [E(RM) - Rf] = Eqm. Price of risk
bM= 1.0
b=
[COV(Ri, RM)/Var(RM)]
CAPM
• 2 Sets of Assumptions:
[1] Perfect market:
• Frictionless, and Perfect information
• No imperfections like tax, regulations, restrictions on short
selling
• All assets are publicly traded and perfectly divisible
• Perfect competition – everyone is a price-taker
[2] Investors:
• Same one-period horizon
• Rational, and maximize expected utility over a meanvariance space
• Homogenous beliefs
CAPM in Details:
What is an equilibrium?
CONDITION 1: Consistent with Individual investor’s Eqm.: Max U
• Assume:
[1] Market is frictionless
=> borrowing rate = lending rate
=> linear efficient set in the return-risk space
[2] Anyone can borrow or lend unlimited amount at risk-free rate
• [3] All investors have homogenous beliefs
=> they perceive identical distribution of expected returns on
ALL assets
=> thus, they all perceive the SAME linear efficient set (we
called the line: CAPITAL MARKET LINE
=> the tangency point is the MARKET PORTFOLIO
NOTE: 2-Fund Separation must hold under the above assumptions.
CAPM in Details:
What is an equilibrium?
CONDITION 1: Individual investor’s equilibrium: Max U
Capital Market Line
E(Rp)
B
Q
E(RM)
Market Portfolio
A
Rf
σM
σp
CAPM in Details:
What is an equilibrium?
CONDITION 2: Demand = Supply for ALL risky assets
• Remember expected return is a function of price.
• Market price of any asset is such that its expected return is just
enough to compensate its investors to rationally hold its outstanding
shares.
CONDITION 3: Equilibrium weight of any risky assets
•
•
•
•
The Market portfolio consists of all risky assets.
Market value of any asset i (Vi) = PixQi
Market portfolio has a value of ∑iVi
Market portfolio has N risky assets, each with a weight of wi
Such that
wi = Vi / ∑iVi for all i
CAPM in Details:
What is an equilibrium?
CONDITION 4: Aggregate borrowing = Aggregate lending
• Risk-free rate is not exogenously given, but is determined by
equating aggregate borrowing and aggregate lending.
CAPM in Details:
What is an equilibrium?
Two-Fund Separation:
Given the assumptions of frictionless market, unlimited lending and
borrowing, homogenous beliefs, and if the above 4 equilibrium
conditions are satisfied, we then have the 2-fund separation.
TWO-FUND SEPARATION:
Each investor will have a utility-maximizing portfolio that is a
combination of the risk-free asset and a portfolio (or fund) of risky
assets that is determined by the Capital market line tangent to the
investor’s efficient set of risky assets
Analogy of Two-fund separation
Fisher Separation Theorem in a world of certainty. Related the two
separation theorems to help your understanding.
CAPM in Details:
What is an equilibrium?
Two-fund separation
Capital Market Line
E(Rp)
B
Q
E(RM)
Market Portfolio
A
Rf
σM
σp
The Role of Capital Market
U’’ U’
E(rp)
Efficient set
P
Endowment Point
σp
The Role of Capital Market
E(rp)
U’’’ U’’ U’
Capital Market Line
U-Max Point
Efficient set
P
M
Endowment Point
Rf
σp
Derivation of CAPM
• Using equilibrium condition 3
wi = Vi / ∑iVi for all i
wi =
market value of individual assets (asset i)
-----------------------------------------------market value of all assets (market portfolio)
• Consider the following portfolio:
hold a (in %) in asset i
and (1-a) (in %) in the market portfolio
Derivation of CAPM
• The expected return and standard deviation of such a
portfolio can be written as:
E(Rp) = aE(Ri) + (1-a)E(Rm)
(Rp) = [ a2i2 + (1-a)2m2 + 2a (1-a) im ] 1/2
• Since the market portfolio already contains asset i and,
most importantly, the equilibrium value weight is wi
• therefore, the “a” in the above equations represent
excess demands for a risky asset
• We know from equilibrium condition 2 that in equilibrium,
Demand = Supply for all asset.
• Therefore, a = 0 has to be true in equilibrium.
Derivation of CAPM
E(Rp) = aE(Ri) + (1-a)E(Rm)
(Rp) = [ a2i2 + (1-a)2m2 + 2a (1-a) im ] 1/2
• Consider the change in the mean and standard deviation with
respect to the percentage change in the portfolio invested in
asset i
 E( R p )
a
(Rp)
a
=
1
2
2
[ a 2  i2 + (1 - a ) 
2
m
= E( R i ) - E( R m )
+ 2a(1 - a) 
im
]
- 1/2
* [ 2a  i2 - 2 
2
m
+ 2a 
2
m
+ 2
im
- 4a 
im
]
• Since a = 0 is an equilibrium for D = S, we must evaluate these
partial derivatives at a = 0
 E( R p )
a
= E( R i ) - E( R m )
(Rp)
a
=
2
 im -  m
m
(evaluated at a = 0)
(evaluated at a = 0)
Derivation of CAPM
• the slope of the risk return trade-off evaluated at point M in
market equilibrium is  E( R )/  a E( R ) - E( R )
=
(evaluated at a = 0)
  ( R )/  a
 -
p
i
m
2
m
im
p

m
• but we know that the slope of the opportunity set at point M must
also equal to the slope of the capital market line, which is given
as:
E( R m ) - R f

m
• Therefore, setting the slope of the opportunity set equal to the
slope of the capital market line
E( R i ) - E( R m )
(
im
-
2
m
) / 
=
m
E( R m ) - R f

m
• rearranging,
E( R i ) = R f +
 im
m
2
[E( R m ) - R f ]
Derivation of CAPM
• From previous page
• Rearranging
E( R i ) = R f +
 im

2
m
[E( R m ) - R f ]
E( R i ) = R f + [E( R m ) - R f ] b i
CAPM Equation
• Where
bi=
 im
m
2
=
COV( R i , R m )
VAR( R m )
E(return) = Risk-free rate of return + Risk premium specific to asset i
E(Ri) = Rf + (Market price of risk)x(quantity of risk of asset i)
Pictorial Result of CAPM
E(Ri)
Security Market
Line
E(RM)
Rf
slope = [E(RM) - Rf] = Eqm. Price of risk
bM= 1.0
b=
[COV(Ri, RM)/Var(RM)]
Properties of CAPM
•
In equilibrium, every asset must be priced so that its riskadjusted required rate of return falls exactly on the security
market line.
•
Total Risk = Systematic Risk + Unsystematic Risk
Systematic Risk – a measure of how the asset co-varies with
the entire economy (CANNOT be diversified away costlessly)
e.g., interest rate, business cycle
Unsystematic Risk – idiosyncratic shocks specific to asset i,
(CAN be diversified away costlessly)
e.g., loss of key contract, death of CEO
•
CAPM quantifies the systematic risk of any asset as its β
•
Expected return of any risky asset depends linearly on its
exposure to the market (systematic) risk, measured by β.
•
Assets with a higher β require a higher risk-adjusted rate of
return. In other words, in market equilibrium, investors are only
rewarded for bearing the market risk.
Use”s” of CAPM
• For valuation of risky assets
• For estimating required rate of return of risky
projects
• As you can see from stocks quotations, beta is a
prominent measure everywhere. The usage of
CAPM is wide-spreaded. Think of other uses of
CAPM as an exercise for yourself. Do some
research on it to help yourself understand more.
Empirical Tests on CAPM
• In the next lecture, we’ll go over some of the empirical
tests of CAPM.
• Think about the following questions:
[1] What are the predictions of the CAPM?
[2] Are they testable?
[3] What is a regression?
[4] How to test hypothesis? What is t-test?