RFS Advance Access originally published online on May 15, 2006
Review of Financial Studies 2007 20(1):41-81; doi:10.1093/rfs/hhl003
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Portfolio Selection with Parameter and Model Uncertainty: A Multi-Prior Approach
University of Texas at Austin
London Business School and CEPR
University of British Columbia and CCFR
Address correspondence to Lorenzo Garlappi, McCombs School of Business, University of Texas at Austin, Austin, TX 78712, or e-mail: lorenzo.garlappi{at}mccombs.utexas.edu.
We develop a model for an investor with multiple priors and aversion to ambiguity. We characterize the multiple priors by a "confidence interval" around the estimated expected returns and we model ambiguity aversion via a minimization over the priors. Our model has several attractive features: (1) it has a solid axiomatic foundation; (2) it is flexible enough to allow for different degrees of uncertainty about expected returns for various subsets of assets and also about the return-generating model; and (3) it delivers closed-form expressions for the optimal portfolio. Our empirical analysis suggests that, compared with portfolios from classical and Bayesian models, ambiguity-averse portfolios are more stable over time and deliver a higher out-of sample Sharpe ratio. (JEL G11)
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