Mean-Variance Optimizers in Asset Allocation
One approach to optimizing asset allocation is to use a mean-variance approach, which seeks allocations that maximize returns for a given level of volatility. These approaches select from various asset classes to combine them into a portfolio that maximizes risk-adjusted return. In an unconstrained mean-variance optimizer the only condition is that the asset class weights sum to one. In a sign-constrained optimizer, there can be no negative weights (short sales) assigned to an asset class.
Unconstrained Mean-Variance Optimization
In an unconstrained optimization, if the weights of any two minimum-variance portfolios are known, the weights of any other can be derived. For example, consider a world of three asset classes – A, B and C.
- The weights assigned to a minimum-variance portfolio X with an expected return of 11% are 70% A, 20% B and 10% C.
- The weights assigned to a minimum-variance portfolio Y with an expected return of 8% are 50% A, 25% B and 25% C.
To find the weights assigned to the minimum variance portfolio with expected return of 9% is found as:
- 9% = 11%(X) + 8%(Y)
- X = 0.67 and Y = 0.33
To find the weights of the individual assets:
- A = 0.67(0.7) + 0.33(0.5) = 0.634 = 63.4%
- B = 0.67(0.2) + 0.33(0.25) = 0.2165 = 21.65%
- C = 0.67(0.1) + 0.33(0.25) = 0.1495 = 14.95%
- 63.4% + 21.65% + 14.95% = 100%, so the equation checks
Sign Constrained Mean Variance Optimization
Given the constraint of no negative asset class weights, one finds that certain portfolios along the efficient frontier will be identified by holding identical sets of asset classes (in varying weights) and that the rate of change in asset weights moving from one portfolio to another is constant. At the intersections (where the set of asset classes change) are “corner portfolios.”
Much like the determination of a minimum-variance portfolio in an unconstrained mean-variance observation, if two adjacent corner portfolios are known the weights of any portfolio between the two can be computed as a weighted average of the two corner portfolios.
Posted on 22nd November 2007
Under: Asset Allocation, FInancial Planning, Institutional Investing, Investment Returns, Portfolio Management | No Comments »


