Money, Banking & Finance Lecture 3 Risk, Return and Portfolio Theory
Aims Explain the principles of portfolio diversification Demonstrate the construction of the efficient frontier Show the trade-off between risk and return Derive the Capital Market Line (CML) Show the calculation of the optimal portfolio choice based on the mean and variance of portfolio returns.
Overview Investors choose a set of risky assets (stocks) plus a risk-free asset. The risk-free asset is a term deposit or government Treasury bill. Investors can borrow or lend as much as they like at the risk-free rate of interest. Investors like return but dislike risk (risk averse).
Preferences of Expected return and risk We have seen how expected return is defined in Lecture 2. The investor faces a number of stocks with different expected returns and differ from each other in terms of risk. The expected return on the portfolio is the weighted mean return of all stocks. First moment. Risk is measured in terms of the variance of returns or standard deviation. Second moment. Investor preferences are in terms of the first and second moments of the distribution of returns.
Return and risk How do return and risk vary relative to each other as the investor alters the proportion of each of the assets in the portfolio? Assume that returns, risk and the covariance are fixed and simply vary the weights in the portfolio. Let E(R 1 )=8.75% and E(R 2 )=21.25 Let w 1 =0.75 and w 2 =0.25 E(R p )=.75x8.75+.25x21.25=11.88 σ 1 =10.83, σ 2 =19.80, ρ 1,2 =-.9549
Portfolio Risk σ 2 p =(0.75) 2 x(10.83) 2 +(0.25) 2 x(19.80) 2 +2x(0.75)x(0.25)x(-0.95)x(10.83)x(19.80) =13.7 σ p =13.7=3.7 Calculate risk and return for different weights
Portfolio risk and return Equity 1Equity 2E(R p )Risk Statew1w2 1108.75%10.83% 20.750.2511.88%3.70% 30.5 15%5% 40121.2519.8%
Locus of risk-return points Expected return Risk=standard deviation (0,1) (.5,.5) (.75,.25) (1,0)
Risk – return locus Can see that the locus of risk and returns vary according to the proportions of the equity held in the portfolio. The proportion (0.75,0.25) is the lowest risk point with highest return. The other points are either higher risk and higher return or low return and high risk. The locus of points vary with the correlation coefficient and is called the efficient frontier
Choice of weights How does the portfolio manager choose the weights? That will depend on preferences of the investor. What happens if the number of assets grows to a large number. If n is the number of assets then will need n(n-1)/2 covariances - becomes intractable A short-cut is the Single Index Model (SIM) where each asset return is assumed to vary only with the return of the whole market (FTSE100, DJ, etc). For n assets the efficient frontier defines a bundle of risky assets.
How is the efficient frontier derived? The shape of the efficient frontier will depend on the correlation between the asset returns of the two assets. If the correlation is ρ = +1 then the portfolio risk is the weighted average of the risk of the portfolio components. If the correlation is ρ = -1 then the portfolio risk can be diversified away to zero When ρ < +1 then not all the total risk of each investment is non-diversifiable. Some of it can be diversified away
Risk-free asset Lets introduce a risk-free asset that pays a rate of interest R f. The rate R f is known with certainty and has zero variance and therefore no covariance with the portfolio. Such a rate could be a short-term government bill or commercial bank deposit.
One bundle of risky assets Take one bundle of risky assets and allow the investor to lend or borrow at the safe rate of interest. The investor can; Invest all his wealth in the risky bundle and undertake no lending or borrowing. Invest less than his total wealth in the single risky bundle and the rest in the risk-free asset. Invest more than his total wealth in the risky bundle by borrowing at the risk-free rate and hold a levered portfolio. These choices are shown by the transformation line that relates the return on the portfolio with one risk-free asset and risk.
Linear Opportunity set Let the risk-free rate R f = 10% and the return on the bundle of assets R N = 22.5%. The standard deviation of the returns on the bundle σ N = 24.87%. The weights on the risky bundle and the risk-free asset can be varied to produce a range of new portfolio returns.
Portfolio Risk and Return StateT-billEquityE(R p )σpσp (1-φ)φ 11010%0% 20.5 16.25%12.44% 30122.5%24.87% 4-0.51.528.75%37.31%
Transformation line The transformation line describes the linear risk- return relationship for any portfolio consisting of a combination of investment in one safe asset and one bundle of risky assets. At every point on a given transformation line the investor holds the risky assets in the same fixed proportions of the risky portfolio ω i.
Transformation line RfRf No lending all investment in bundle E(R p ) σpσp All lending 0.5 lending + 0.5 in risky bundle -0.5 borrowing + 1.5 in risky bundle
A riskless asset and a risky portfolio An investor faces many bundles of risky assets (eg from the London Stock Exchange). The efficient frontier defines the boundary of efficient portfolios. The single risky asset is replaced by a risky portfolio. We can find a dominant portfolio with the riskless asset that will be superior to all other combinations.
Combining risk-free and risky portfolios A B C RfRf E(R p ) σpσp
Borrowing and Lending The investor can lend or borrow at the risk- free rate of interest rate. The risk-free rate of interest R f represents the rate on Treasury Bills or some other risk-free asset. The efficiency boundary is redefined to include borrowing.
Borrowing and lending frontier E(R p ) σpσp RfRf A B C
Combined borrowing and lending at different rates of interest The investor can borrow at the rate of interest R b Lend at the rate of interest R f The borrowing rate is greater than the risk- free rate. R b > R f Preferences determine the proportions of lending or borrowing,
Combining borrowing and lending E(R p ) σpσp RbRb A B C D RfRf P Q
Separation Principle Investor makes 2 separate decisions Given knowledge of expected returns, variances and covariances the investor determines the efficient frontier. The point M is located with reference to R f. The investor determines the combination of the risky portfolio and the safe asset (lending) or a leveraged portfolio (borrowing).
Market portfolio and risk reduction Portfolio risk Diversifiable risk Non- diversifiable risk Number of securities 20
Summary We have examine the theory of portfolio diversification We have seen how the efficient frontier is constructed. We have seen that portfolio diversification reduces risk to the non-diversifiable component.