Biases in variance of decomposed portfolio returns

摘要

Significant portfolio variance biases arise when contrasting multiperiod portfolio returns based on the assumption of fixed continuously rebalanced portfolio weights as opposed to buy-and-hold weights. Empirical evidence obtained using S&P 500 constituents from 2003 to 2011 demonstrates that, compared with a buy-and-hold assumption, applying fixed weights led to decreased estimates of portfolio volatilities during 2003, 2005 and 2010, but caused a significant increase in volatility estimates in the more turbulent 2008 and 2011. This discrepancy distorts assessments of portfolio risk-adjusted performance when inappropriate weight assumptions are employed. Consequently, these variance biases have effect on statistical inference in factor models and may result in erroneous portfolio size recommendations for adequate diversification.