Modern Portfolio Theory
Modern portfolio theory “Portfolio analysis” redirects here. For theorems about where Rp is the return on the port- the mean-variance efficient frontier, see Mutual fund folio, Ri is the return on asset i and separation theorem. For non-mean-variance portfolio wi is the weighting of component analysis, see Marginal conditional stochastic dominance. asset i (that is, the proportion of as- set “i” in the portfolio). Modern portfolio theory (MPT), or mean-variance • Portfolio return variance: analysis, is a mathematical framework for assembling a P σ2 = w2σ2 + portfolio of assets such that the expected return is maxi- Pp P i i i w w σ σ ρ , mized for a given level of risk, defined as variance. Its key i j=6 i i j i j ij insight is that an asset’s risk and return should not be as- where σ is the (sample) standard sessed by itself, but by how it contributes to a portfolio’s deviation of the periodic returns on overall risk and return. an asset, and ρij is the correlation coefficient between the returns on Economist Harry Markowitz introduced MPT in a 1952 [1] assets i and j. Alternatively the ex- essay, for which he was later awarded a Nobel Prize in pression can be written as: economics. P P 2 σp = i j wiwjσiσjρij , where ρij = 1 for i = j , or P P 1 Mathematical model 2 σp = i j wiwjσij , where σij = σiσjρij is the (sam- 1.1 Risk and expected return ple) covariance of the periodic re- turns on the two assets, or alterna- MPT assumes that investors are risk averse, meaning that tively denoted as σ(i; j) , covij or given two portfolios that offer the same expected return, cov(i; j) .
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