This paper examines the degree of portfolio inefficiency subject to various constraints on portfolio weights. When portfolio weights are unconstrained, the posterior loss in expected return on the NYSE-AMEX market portfolio is over 20% (annualized). In contrast, when portfolio weights are constrained to be nonnegative, the posterior loss in expected return is only about 4% (annualized). In addition, short-sale constraints greatly reduce uncertainty in inferences about portfolio efficiency.
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Zhenyu Wang (1998) studied this question.
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