The Why and How of Mutual Fund Standard Deviations

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To the interested observer, mutual fund standard deviations raise two tantalizing questions: Are standard deviations relevant when funds, by definition, eliminate the unsystematic component of total risk? and How can two respected giants in the investments field like Fidelity and Morningstar use the same returns, intervals and measurement period for the same fund and end up with glaringly different standard deviations? To answer the question of relevance, we recall Evans and Archer's (1968) argument that as much as 90 percent of a portfolio's unsystematic risk can be diversified away with 12 to 18 stocks. Since that diversifiable risk is a component of standard deviations, it follows that they would tend to overstate the risk of well-diversified portfolios.

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