Merton's [26] recent extension of the CAPM proposed that asset returns are an
increasing function of their beta risk, residual risk, and size and a decreasing function
of the public availability of information about them. Associating the latter with asset
liquidity and following Amihud and Mendelson's [2] proposition that asset returns
increase with their illiquidity (measured by the bid-ask spread), we jointly estimate the
effects of these four factors on stock returns.