Biases in Computed Returns: An Application to the Size Effect (Revision of 2-83)

Previous estimates of a "size effect" based on daily returns data are biased. Several properties of quoted closing prices impart an upward bias to computed returns on individual stocks. Returns computed for buy-and-hold portfolios largely avoid the bias induced by closing prices. Based on such buy-and-hold returns, the full-year size effect is half as large as previously reported, and all of the full-year effect is, on average, due to the month of January.