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Two of the most important concepts in investing are return and risk. If returns of securities are viewed as random variables, then the reward of a portfolio can be measured as the expectation, or mean, of the portfolio return and its risk can be measured as the variance of the portfolio return. Of course, many other approaches are possible, but mean-variance
analysis is one of the simplest and most powerful. This approach was suggested by Harry Markowitz, who won the Nobel Prize in economics 1990 for his work in this area in the late 1950’s and early 1960’s. For a history, discussion, and extensive bibliography, see Markowitz (1987).