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Portfolio optimisation

Miquel Noguer i Alonso, Daniel Bloch and David Pacheco Aznar

13.1 INTRODUCTION

Portfolio optimisation is about computing the optimal weights in a portfolio of assets for a given investment horizon by maximising some objective function. The modern portfolio theory (MPT) (Markowitz 1952a, 1959) entails a two-step construction process.

  1. Estimate population moments. Compute the first two moments of asset returns over a given horizon; this is a prediction problem for the vector of expected returns and covariance matrix.

  2. Portfolio construction. Optimise a mean–variance criteria over possible combinations of assets; this is a quadratic optimisation problem.

Researchers and practitioners have devised several techniques for estimating expected returns and the covariance matrix (James and Stein 1961; Black and Litterman 1992). However, it is extremely difficult to estimate returns accurately because of the lack of a long time series of data (Merton 1980). Further, estimates of variance-covariance are seldom well behaved. This difficulty in estimating the first two moments of returns leads to the error-maximising problem (EMP) of the mean–variance portfolio (Best and Grauer 1991). That is, we get unstable and extremely positive and negative weights in step

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