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FINS5513 Lecture 2
Forming Optimal Portfolios 2❑ 2.1 Optimal Risky Asset Portfolio Construction ➢ Minimum Variance Frontier ➢ Efficient Frontier ➢ Optimal portfolio: no risk-free asset ❑ 2.2 Introducing the Risk-Free Asset ➢ Combining Risky and Risk-Free Assets: Complete portfolio ➢ Capital Allocation Line ❑ 2.3 Deriving Optimal Portfolio Weights ➢ Optimal Risky Portfolio ∗ ➢ Optimal Complete Portfolio ∗ ➢ Separation Theorem ➢ Leveraged Portfolios: kinked CAL Lecture Outline 2.1 Optimal Risky Asset Portfolio Construction FINS5513 4Minimum Variance Frontier FINS5513 5❑ Imagine an investment universe with assets characterized by the risk-return profile as plotted below: ❑ So how do we form an optimal portfolio out of these 10 individual assets What is an Optimal Portfolio? 6❑ By combining risky assets in a portfolio in different proportions we can construct portfolios with risk-return profiles which are on the red line below ❑ The red line is known as the minimum variance frontier MVF – the lowest risk portfolio at each level of return. The derivation steps are: 1. Set the target expected portfolio return (e.g., 10%) 2. Optimise portfolio weights to minimise variance at this level of return 3. Repeat at different return levels till we have plotted the frontier Minimum Variance Frontier g Mathematically, we are solving following constrained optimization problem: min = ( =1 ) Subject to the constraint: = =1
= 7❑ From the mean-variance criterion, we know that any portfolio or asset combination below the MVF will be dominated by a portfolio on the MVF ❑ Similarly, any portfolio below the turning point of the MVF will be dominated by one above the turning point ➢ The turning point is the portfolio (asset combination) that has the lowest possible risk level. It is known as the Global Minimum Variance Portfolio (GMVP) Minimum Variance Frontier g 8❑ We can see that not every point on the MVF is optimal. Those points below the turning point of the frontier are dominated by points above the turning point (higher return for the same level of risk) ❑ The GMVP is the turning point, and it is the portfolio combination which provides the lowest possible risk (defined by standard deviation). It is a unique set of portfolio asset weightings which results in the lowest possible standard deviation: ❑ For a portfolio with just 2 risky assets (or asset classes), the GMVP portfolio weights are given by: ➢ 1 = 2 2−(1,2) 1 2+2 2−2(1,2) ➢ 2 = 1 − 1 Global Minimum Variance Portfolio (GMVP) 9Efficient Frontier FINS5513 10 ❑ As each portfolio on the MVF above the GMVP dominates each portfolio below the GMVP, we discard the portion below the GMVP ❑ The un-dominated portfolios on the MVF(i.e., the upper-half of MVF) make up the Efficient Frontier ➢ To plot the efficient frontier we would need to identify the GMVP first ➢ All portfolio combinations on the MVF below the GMVP are discarded as they are all dominated by the GMVP ➢ Risk averse investors should only choose portfolios on the efficient frontier Efficient Frontier E(r) σ mvp GMVP 11 E(r) σ ❑ So, where along the efficient frontier should we invest? ❑ We should pick the asset portfolio weighting combination which provides the highest utility ➢ Highest utility is represented by the highest attainable indifference curve ➢ The portfolio marked by the star gives the highest possible utility for this investor ➢ It lies on the tangent point between the efficient frontier and the highest attainable indifference curve Optimal Point on the Efficient Frontier? Optimal risky portfolio 12 ❑ Although all investors face the same efficient frontier, they will have different utility functions (and thus different indifference curves). ➢ So the optimal risky portfolio may differ between investors ❑ So far, we only consider about the risky assets. What will happen if we add the risk-free asset into the model? How does it affect the optimal investment choice? Optimal Portfolio: No Risk-free Asset E(r)
Optimal risky portfolio Investor B Optimal risky portfolio Investor A 2.2 Introducing the Risk-Free Asset FINS5513 14 ❑ Now let’s add a risk-free asset: ➢ Short-term Government bills (T-bills) are often considered as the risk-free asset, as they have almost no default risk and limited interest rate risk ❑ The return on the risk-free asset (the “risk-free rate”) is denoted . By definition of risk free, we have: ➢ = ➢ = ➢ , = ❑ Even if we don’t take risk, we still want a positive return. Hence is generally positive ❑ The risk premium on a risky asset is: – ➢ Note that excess return/risk premium is denoted while total return is denoted The Risk-Free Asset 15 Combining Risky and Risk-Free Assets FINS5513 16 ❑ One of the most important capital allocation decisions is determining how much of our portfolio to allocate to risk-free assets vs risky assets ➢ Let’s now combine a risky asset portfolio with a risk-free asset ❑ First, let’s denote all asset class returns and risks correctly: ➢ We have identified an efficient risky portfolio on the efficient frontier, and denote it Combining Risky and Risk-Free Assets Risk-Free Assets ▪ T-Bills/Govt Bonds ▪ Money Market Funds ▪ Bank Deposits Expected Return () Risk = 0 Weighting (1 − ) Combination Complete Portfolio C Comb. Expected Return () = 1 − + () = + [() – ] Comb. Risk = Risky Assets ▪ Equities: wE1,wE2,wE3… wEn ▪ Risky bonds: wB1,wB2,wB3… wBn ▪ Alternatives: wA1,wA2,wA3… wAn 17 ❑ What we are trying to derive is the appropriate weighting between the risky assets portfolio and the risk-free asset(s) in our Complete Portfolio ❑ As usual, to determine weightings, we must first derive the expected return and risk of ❑ The return on our complete portfolio is: = 1 − + ❑ So, the expected return on the Complete Portfolio is: () = 1 − + () ➢ It is useful to express this equation as a risk premium. Rearranging we have: () = + [() – ] [() – ] is often referred to as the risk premium Complete Portfolio Expected Return 18 ❑ What about risk? We know that portfolio risk for a 2-asset portfolio ( and ) is given by: 2 = 2 2 + (1 − )2 2 + 2 1 − , ❑ However, we know is a constant. Therefore, we have 2 = 0 ; , = 0 ❑ So, the portfolio risk equation above simplifies to: 2 = 2 2 ⇒ = ❑ Now, we have: ቐ () = + [() – ] = ⇒ = ⇒ () = + [() – ] Complete Portfolio Risk 19 Capital Allocation Line FINS5513 20 ❑ Let’s look at this equation carefully: () = + [() – ] ➢ It is a linear line in the − space with intercept and slope equal to the Sharpe ratio: − ❑ So graphically, if we drew a straight line connecting to the risky asset : ➢ This line is known as the Capital Allocation Line (CAL) for risky asset : ➢ The slope of the line will equal ’s Sharpe ratio ➢ (the weight in risky assets) will tell us where along the line we sit Graphical Representation 21 ❑ The higher is , the more we allocate to risky assets, and therefore the higher the expected return and the risk of the combined portfolio ❑ If > 1, it means borrowing at (instead of investing in the risk-free asset at ) and investing the proceeds into risky assets (i.e., taking a levered position in risky assets) ❑ We can label the line that links with risky portfolio – the Capital Allocation Line for Capital Allocation Line = 1 is the -intercept as = 0 E(r) σ rf P CALP C ( y=0.25) C ( y=0.75) C ( y=0.5) C ( y=1.25) 22 Low Risk Fund has identified an efficient portfolio of risky assets on the efficient frontier with an expected return () = 9.21% and risk of = 16.92% a) What is the Sharpe ratio of ? b) If the fund prefers a lower risk level, how would it efficiently combine risky portfolio and the risk-free asset ( = 2.0%) to create a complete portfolio with risk level of = 12%? c) What is the expected return () and Sharpe ratio of this complete portfolio ? Example: Risky and Risk-Free Assets