Portfolio Managment 3-228-07 Albert Lee Chun Construction of Portfolios: Introduction to Modern Portfolio Theory Lecture 3 16 Sept 2008 0 Course Outline Sessions 1 and 2 : The Institutional Environment Sessions 3, 4 and 5: Construction of Portfolios Sessions 6 and 7: Capital Asset Pricing Model Session 8: Market Efficiency Session 9: Active Portfolio Management Session 10: Management of Bond Portfolios Session 11: Performance Measurement of Managed Portfolios 1 Portfolio Risk as a Function of the Number of Stocks in the Portfolio Albert Lee Chun Portfolio Management 7-2 2 Portfolio Diversification Albert Lee Chun Portfolio Management 7-3 3 Two-Security Portfolio: Return rP w1 r1 w2 r2 w1 w2 r1 r2 = proportion of funds in Security 1 = proportion of funds in Security 2 = expected return on Security 1 = expected return on Security 2 n w i 1 i 1 Albert Lee Chun Portfolio Management 7-4 4 Two-Security Portfolio: Risk 2 2 2 1 1 p w 2 2 w2 2 2w1w2Cov(r1, r2) 12 = variance of Security 1 22 = variance of Security 2 Cov(r1,r2) = covariance of returns for Security 1 and Security 2 Albert Lee Chun Portfolio Management 7-5 5 Covariance Cov(r1, r2) 1,2 1 2 1,2 = Correlation coefficient of returns 1 = Standard deviation of returns for Security 1 2 = Standard deviation of returns for Security 2 Albert Lee Chun Portfolio Management 7-6 6 Correlation Coefficients: Possible Values Range of values for 1,2 + 1.0 > > -1.0 If = 1.0, the securities would be perfectly positively correlated If = - 1.0, the securities would be perfectly negatively correlated Albert Lee Chun Portfolio Management 7-7 7 Three-Security Portfolio rp w1r1 w2r2 w3r3 p w1 1 w2 2 w3 3 2 2 2 2 2 2 2 2 w1w2Cov(r1 , r2 ) 2 w1w3Cov(r1 , r3 ) 2 w2 w3Cov(r2 , r3 ) Albert Lee Chun Portfolio Management 7-8 8 Generally, for an n-Security Portfolio: rp n w i i 1 2 p Albert Lee Chun n w i 1 ri n 2 w jwkCov(rj , rk ) 2 2 i i j,k 1 jk Portfolio Management 7-9 9 Review of Portfolio Statistics N E ( R p ) wi E ( Ri ) i 1 N 2 2 = wi i + 2 p i=1 i , j Albert Lee Chun N N w w i j Cov ( r i , r j ) i j i=1 j=1 Cov ( ri , r j ) i j Cov(ri , rj ) i , j i j Portfolio Management 10 Today’s Lecture Utility Functions, Indifference Curves Capital Allocation Line Minimum Variance Portfolios Optimal Portfolios in a 2 security world (1 risk-free and 1 risky) 2 security world (2 risky) 3 security world (2 risky and 1 risk-free) N security world (with and without risk-free asset) Albert Lee Chun Portfolio Management 11 Utility Functions Albert Lee Chun Portfolio Management 12 Risk Aversion Given a choice between two assets with equal rates of return, risk-averse investors will select the asset with the lower level of risk. Risk-averse investors need to be compensated for holding risk. The higher rate of return on a risky asset i is determined by the risk-premium: E(Ri) – Rf. Albert Lee Chun Portfolio Management 13 Example: Risk Premium W1 = $150 Profit = $50 Risky Investment 1-p = .4 100 W2 = $80 Profit = -$20 Profit = $5 T-bills Expected return: (50%)(.6) + (-20%)(.4) = 22% Risk Premium = E(Ri) – Rf = 22% - 5% = = 17% Albert Lee Chun Portfolio Management 14 Measure of Investor Preferences A utility function captures the level of satisfaction or happiness of an investor. The higher the utility, the happier the investors. For example, if investor utility depends only of the mean (let µ= E(R)) and variance (2) of returns then it can be represented as a function: U = f ( µ, ) The locus of portfolios that provide the same level of utility for an investor defines an indifference curve. Albert Lee Chun Portfolio Management 15 Example: An Indifference Curve U=5 (Rp) U=5 The investor is indifferent between X and Y, as well as all points on the curve. All points on the curve have the same level of utility (U=5). Albert Lee Chun Portfolio Management 16 Direction of Increasing Utility Expected Return U3 U2 U3 > U2 > U1 U1 Direction of Increasing Utility Standard Deviation Albert Lee Chun Portfolio Management 17 Two Different Investors Expected Return U3’ U2’ U1’ Which investor is more risk averse? U3 U2 U1 Standard Deviation Albert Lee Chun Portfolio Management 18 Quadratic Utility The utility of the investor is quadratic if only the mean and variance of returns is important for the investor. 1 U E ( R) AE ( R E ( R)) 2 2 1 A 2 2 A is a constant that determines the degree of risk aversion: it increases with the risk-aversion of the investor. (Note that the 1/2 is just a normalizing constant.) Note that A > 0, implies that investors dislike risk. The higher the variance the lower the utility. Albert Lee Chun Portfolio Management 19 Indifference Curves Let’s look at an example of points on an indifference curve for an investor with a quadratic utility function. Note that higher variance is accompanied by a higher rate of return to compensate the risk-averse nature of the investor. E(Rp) 0.10 0.15 0.20 0.25 (Rp) 0.200 0.255 0.300 0.339 Utility = E(Rp) – ½ A×VAR(Rp) 0.10 – ½ 4 0.2002 = 0.02 0.15 – ½ 4 0.2552 = 0.02 0.20 – ½ 4 0.3002 = 0.02 0.25 – ½ 4 0.3992 = 0.02 Quadratic utility function with A = 4. Albert Lee Chun Portfolio Management 20 Certain Equivalent The certain equivalent is the risk-free (certain) rate of return that offers investors the same level of utility as the risky rate of return. The investor is indifferent between a risky return and it’s certain equivalent. Example: Suppose an investor has quadratic utility with A = 2. A risky portfolio offers an E(R) equal to 22% and standard deviation 34%. The utility of this portfolio is: U = 22% - ½×2×(34%)² = 10.44% The certain equivalent is equal to 10.44% because the utility of obtaining a certain rate of return of 10.44% is U = 10.44% - ½ × 2×(0%)² = 10.44% Albert Lee Chun Portfolio Management 21 Risk-Neutral Indifference Curves E(RP) U3 > U2 > U1 U4 U3 U2 U1 P Neutral attitude toward risk. Investor is indifferent between different levels of standard deviation. Albert Lee Chun Portfolio Management 22 Slope of the Indifference Curve A steep indifference curve coincides with strong riskaversion. The slope of the indifference curve captures the required compensation for each unit of additional risk. This compensation is measured in units of expected return for each unit of standard deviation. High risk-aversion implies a high degree of compensation for taking on an additional unit of risk and is represented by a steep slope. Albert Lee Chun Portfolio Management 23 Risk-Averse Indifference Curves U4 E(RP) U3 Expected Return U2 U1 U3 > U2 > U1 P Standard Deviation Albert Lee Chun Portfolio Management 24 Two Different Investors Expected Return More risk averse U3’ U2’ Less risk averse U1’ Which investor is more risk averse? U3 U2 U1 Standard Deviation Albert Lee Chun Portfolio Management 25 Stochastic Dominance Prefers any portfolio in Z1 to X. The rankings between portfolios in Z2 or Z3 and X, depends on the preferences of the investor! Prefers X to any portfolio in Z4. Albert Lee Chun Portfolio Management 26 Imagine a world with 1 risk-free security and 1 risky security Albert Lee Chun Portfolio Management 27 1 Risk-Free Asset and 1 Risky Asset Suppose we construct a portfolio P consisting of 1 risk-free asset f and 1 risky asset A: E (rp ) w f r f w A E (rA ) =w 00 2 p 2 A 2 A p = wA A Note: The variance of the risk-free asset is 0, and the covariance between a risky asset and a risk free asset is naturally equal to 0. Albert Lee Chun Portfolio Management 28 1 Risk Free Asset and 1 Risky Asset Suppose WA = .75 E(rA) = 15% E(rP) = 13% P rf = 7% f 0 P =16.5% A =22% E(rP) = .25*.07+.75*15=13% Albert Lee Chun A p = .75*.22 = 16.5% Portfolio Management 29 Capital Allocation Line (CAL) Equation of CAL E (rP ) Line E(rA) A A E(rp) P rf * P rf Slope of CAL E( r A ) - r f A f Intercept 0 Albert Lee Chun E (rA ) r f p Portfolio Management A 30 Maximize Investor Utility In our world with 1 risk free asset and 1 risky asset, if an investor has quadratic utility, what is the optimal portfolio allocation? Utility: U E (rP ) 1 2 A AP Expected return E (rP ) wE (rA ) (1 w)r f , and variance: 2 w 2 2 P A Goal is to Maximize utility. How? Albert Lee Chun Portfolio Management 31 Normally a Bear Lives in a Cave, that is Concave, A concave function has a negative second derivative. then to find the top of the cave (i.e. or to maximize a concave function), take the first derivative and set it equal to 0: Albert Lee Chun Portfolio Management 32 However, if the Bear is Swimming in a Bowl, that is Convex, A convex function has a positive second derivative. Then to find the bottom of the bowl (i.e. or to minimize a convex function), take the first derivative and set equal to 0: Albert Lee Chun Portfolio Management 33 Maximize Investor Utility U E (rP ) 1 2 A P2 wE (rA ) (1 w)r f - 1 Aw A 2 2 2 Take derivative of U with respect to w and set equal to 0: dU ( w) E (rA ) r f Aw A2 0 dw E( r A ) - r f * w= A 2A Albert Lee Chun w* is the optimal weight on risky asset A Portfolio Management 34 Example 1 Supppose E(rA) = 15%; (rA) = 22% and rf = 7% and we have a Quadratic investor with A = 4, then w* = (0.15-0.07)/[4*(0.22)2] E( r A ) - r f w= A 2A * = 0.41 His optimal allocation is: 41% of his capital in the risky portfolio A and 59% in the risk-free asset. E(rp) = 0.59*7%+0.41*15%=10.28% and (rp) = 0.41*0.22=9.02% Albert Lee Chun Portfolio Management E (rp ) (1 w*)r f w * E (rA ) p= w* A 35 Example 2 Supppose E(rA) = 15%; (rA) = 22% and rf = 7% and we have a less risk-averse Quadratic investor with A = 1, then w* = (0.15-0.07)/[1*(0.22)2] = 1.65 > 1 This investor should place 165% of his capital in A. He needs to borrow 65% of his capital at the risk free rate of 7%. E(Rp) = 1.65(0.15) + -0.65(0.07)= 20.2% (rp) = 1.65*0.22= 0.363 = 36.3% His utility is: U = 0.202 – 0.5*1*(0.3632) = 0.1361 Albert Lee Chun Portfolio Management 36 Graphical View The optimal allocation along the capital allocation line depends on the risk-aversion of the agent. Risk-seeking agents with w* greater than 1 will borrow at the risk-free rate and invest in security A E(r) A Ex2: Borrower 7% Ex1: Lender A = 22% The optimal allocation is the point of tangency between the CAL and the investor’s utility function. Albert Lee Chun Portfolio Management 37 Different Borrowing Rate What if the borrowing rate is higher than the lending rate? E(r) A 9% 7% A = 22% w* = (0.15-0.09)/[1*(0.22)2] = 1.24 1.24 < 1.65 Albert Lee Chun Portfolio Management 38 Different Borrowing Rate Supppose E(rA) = 15%; (rA) = 22% and rf = 7% lending rate, and a 9% borrowing rate, Quadratic investor with A = 1, then w* = (0.15-0.09)/[1*(0.22)2] = 1.24 1.24 < 1.65 This investor should place 124% of his capital in A. He needs to borrow 24% of his capital at the risk free rate of 9%. This is less than what he would borrow at a 7% borrowing rate. E(Rp) = 1.24(0.15) + -0.24(0.09)= 16.44% (rp) = 1.24*0.22= 27.28% Increasing the borrowing rate, lowers his utility from before: U = 0.1644 – 0.5*1*(0.27282) = .1272 < .1361 Albert Lee Chun Portfolio Management 39 Imagine a world with 2 risky securities Albert Lee Chun Portfolio Management 40 Expected Return and Standard Deviation with Various Correlation Coefficients Albert Lee Chun Portfolio Management 7-41 41 Portfolio Expected Return as a Function of Investment Proportions Albert Lee Chun Portfolio Management 7-42 42 Portfolio Standard Deviation as a Function of Investment Proportions Albert Lee Chun Portfolio Management 7-43 43 Returning to the Two-Security Portfolio E(rp ) w1r1 w2r2 and 2 2 2 1 1 2 2 p w w2 2 2w1w2Cov(r1, r2), or 2 1 2 1 p w 2 2 w2 2 2w1w2Cov(r1, r2) Question: What happens if we use various securities’ combinations, i.e. if we vary ? Albert Lee Chun Portfolio Management 7-44 44 Portfolio Expected Return as a function of Standard Deviation Albert Lee Chun Portfolio Management 7-45 45 Perfect Correlation E(R) 0.20 With two perfectly correlated securities, all portfolios will lie on a straight line between the two assets. 0.15 0.10 0.05 E Rij = +1.00 D With short selling 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11 0.12 Albert Lee Chun Portfolio Management 46 Perfect Correlation = +1 E( RP ) wD E( RD ) wE E( RE ) = w + w + 2 wD wE 1 D E 2 p 2 D 2 D 2 E 2 E Recall that Cov(rE , rD ) E D = ( wD D + wE E ) 2 p = wD D wE E Albert Lee Chun Portfolio Management 47 Zero Correlation E(R) f 0.20 g 2 With uncorrelated h assets it is possible i j to create a two Rij = +1.00 asset portfolio with k lower risk than 1 either asset! Rij = 0.00 0.15 0.10 0.05 - 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11 0.12 Albert Lee Chun Portfolio Management 48 Zero Correlation =0 = w + w + 2 wD wE 0 D E 2 2 2 2 = wD D + wE E 2 p 2 D 2 D 2 E 2 E Recall that Cov(rE , rD ) E D 2 2 2 2 = + p wD D wE E Albert Lee Chun Portfolio Management 49 Positive Correlation E(R) f 0.20 g 2 With positively h correlated assets a i two asset portfolio j Rij = +1.00 lies between the k Rij = +0.50 first two curves 1 Rij = 0.00 0.15 0.10 0.05 2 2 2 2 = + wD D wE E + 2 wD wE DE D E 2 p 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11 0.12 Albert Lee Chun Portfolio Management 50 Negative Correlation E(R) 0.20 0.15 0.10 With negatively correlated assets it is possible to create a portfolio with much lower risk. Rij = -0.50 f 2 g h j i k Rij = +1.00 Rij = +0.50 1 Rij = 0.00 Negative 0.05 2 2 2 2 = + wD D wE E + 2 wD wE DE D E 2 p 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11 0.12 Albert Lee Chun Portfolio Management 51 Perfect Negative Correlation E(R) f 0.20 Rij = -1.00 0.15 0.10 0.05 - 2 g h j i k Rij = +1.00 Rij = +0.50 1 Rij = 0.00 With perfectly negatively correlated assets it is possible to create a two asset portfolio with NO RISK. How? 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11 0.12 Albert Lee Chun Portfolio Management 52 Perfect Negative Correlation = -1 = w + w - 2 wD wE D E 2 p 2 D 2 D 2 E 2 E Note that Cov(rE , rD ) 1 E D = ( wD D - wE E ) 2 p =| wD D - wE E | To get a zerovariance portfolio we need to set: Albert Lee Chun E D and wE = D + E D + E then we obtain P=0 wD = Portfolio Management 53 Minimum Variance Portfolio Albert Lee Chun Portfolio Management 54 Minimum Variance Portfolio Min = w + (1 - wD ) 2E + 2 wD (1 - wD ) DE 2 p 2 D 2 D 2 2p = 2 wD 2D - 2(1 - wD ) 2E + (2 - 4 wD ) DE wD = wD (2 2D + 2 2E - 4 DE ) - (2 2E - 2 DE ) = 0 2 E - DE D E DE min = 2 wD = 2 2 2 + 2 + D E DE D E - 2 DE D E min min = 1 wE wD 2 E Albert Lee Chun Portfolio Management 55 Minimum Variance Portfolio 1> > -1 = -1 w min D = 2D 2E - DE + 2E - 2 DE 2 E ( E + D ) = E E + D E min = = wD 2 2 2 + + 2 D E D E ( D + E ) D + E =0 2 2 0 E E min = wD 2D + 2E - 0 2D + 2E =1 If no short sales, then MVP is equal to the asset with the minimum variance.* (Our formula doesn’t work here, why? Think about the bear in a cave/bowl.) Albert Lee Chun Portfolio Management *With short sales can obtain 0 variance. 56 Portfolio of Two Securities: Correlation Effects • • • • Relationship depends on correlation coefficient -1.0 < < +1.0 The more negative the correlation, the greater the risk reduction potential If= +1.0, no risk reduction is possible (absent short sales). Albert Lee Chun Portfolio Management 7-57 57 Example: MVP Example: Suppose there are 2 securities A an B: E(r) A B 10% 14% 15% 20% A,B 0.2 Find the minimum variance portfolio? 2 B - A B min min = 1 w = 2 w w B A 2 + 2 A B A B min A Albert Lee Chun Portfolio Management 58 Example: MVP Albert Lee Chun Portfolio Management 59 Similar Example from the Book • Suppose our investment universe comprises the two securities of Table 7.1: D E E(r) 8% 13% 12% 20% D,E 0.3 • What are the weights of each security in the minimum-variance portfolio? Albert Lee Chun Portfolio Management 7-60 60 Book Example: MVP • Solving the minimization problem we get: E Cov(rD , rE ) wD D 2 E 2 2Cov(rD , rE ) 2 • Numerically: (20) 2 72 wD 0.82 2 2 (20) (12) 272 wE 1 wD 0.18 Albert Lee Chun Portfolio Management 7-61 61 Investor’s Utility Less Risk Averse Investors U’’’ E(r) U’’ U’ More Risk Averse Investors Albert Lee Chun Portfolio Management 62 Investor’s Utility Maximization U E (r ) 1 2 A 2 E(rP ) wD E(rD ) wE E(rE ) 2 = + (1 w wD ) E + 2 wD (1 - wD ) DE 2 p 2 D 2 D 2 Homework, you should be able to show that the optimal solution is: 2 E( ) E( ) + A( r r D E E - DE ) * wD = A ( 2D + 2E - 2 DE ) Albert Lee Chun Portfolio Management 63 Example Example: Suppose there are only 2 portfolios: E(r) A B 10% 14% 15% 20% A,B 0.2 Find the optimal portfolio for a investor with quadratic utility ( A = 3)? 2 E E + A r r A B B - A B * * * = , 1 wA wB wA 2 2 A A + B - 2 A B Albert Lee Chun Portfolio Management 64 Example * A w = 0.41, - 2 * 0.2 * 0.2 * 0.15 0.10 - 0.14 + 3 0.2 - 0.2 * 0.2 * 0.15 3 0.2 2 2 + 0.15 2 * 1 w w A 0.59 * B Albert Lee Chun Portfolio Management 65 Imagine a world with 2 risky securities and 1 risk-free security Albert Lee Chun Portfolio Management 66 Two Feasible CALs Albert Lee Chun Portfolio Management 7-67 67 Optimal CAL and the Optimal Risky Portfolio Albert Lee Chun Portfolio Management 7-68 68 With a Risk Free Asset E(r) CAL 3 I’m the optimal risky portfolio, the tangent portfolio!! CAL 2 CAL 1 E rf I’m the one that maximizes the slope of the Capital Allocation Line ! D Albert Lee Chun Portfolio Management 69 Optimal Portfolio Weights S p= E( R p ) - r f p E( RP ) wD E( RD ) wE E( RE ) 2 2p = w2D 2D + (1 - wD ) 2E + 2 wD (1 - wD ) DE Homework: If you are ambitious, try to show that the optimal solution is: wD* = E r r D E r r - E r r + E r r E r r D f 2 E 2 E f E f E 2 D f D f DE E r E r f DE w*E 1 w*D Albert Lee Chun Portfolio Management 70 The Optimal Overall Portfolio Albert Lee Chun Portfolio Management 7-71 71 The Proportions of the Optimal Overall Portfolio Albert Lee Chun Portfolio Management 7-72 72 Optimal Overall Portfolio: 2 Investors i and j E(r) j PT Optimal weight for each investor depends on risk aversion parameter A E( R PT ) - r f w= A 2PT i * rf Albert Lee Chun CAL Portfolio Management 73 Different Lending and Borrowing E(r) P2 P1 rfB rf Albert Lee Chun Portfolio Management 74 Now imagine a world with many risky assets Albert Lee Chun Portfolio Management 75 The Markowitz Problem Max E Rp wi E Ri i 1 wi Subject to the constraint N N w w i 1 j 1 N w i 1 Albert Lee Chun i i j ij * p 1 Portfolio Management 76 Efficient Frontier E(R) Albert Lee Chun Efficient Frontier Portfolio Management 77 Extending Concepts to All Securities • • • The optimal combinations result in lowest level of risk for a given return The optimal trade-off is described as the efficient frontier These portfolios are dominant Albert Lee Chun Portfolio Management 7-78 78 Minimum Variance Frontier of Risky Assets E(r) Minimum variance frontier Global minimum variance portfolio Individual assets Albert Lee Chun Portfolio Management 7-79 79 Efficient Variance Frontier of Risky Assets E(r) Efficient frontier Global minimum variance portfolio Individual assets Albert Lee Chun Portfolio Management 7-80 80 The Efficient Portfolio Set Albert Lee Chun Portfolio Management 7-81 81 Now imagine a world with many risky assets and 1 risk-free asset Albert Lee Chun Portfolio Management 82 Capital Allocation Lines Albert Lee Chun Portfolio Management 7-83 83 The Market Portfolio E(R) Capital Market Line M Albert Lee Chun Portfolio Management 84 Readings Readings for Today’s lecture. 1. Chapter 7. 2. If you have not taken Investments, you may want to review Chapter 6 as well. Readings for next week: Finish reading Chapter 7, including appedicies. Readings for the week after next: (Course Reader) Other Portfolio Selection Models Albert Lee Chun Portfolio Management 85