The foundation of modern portfolio theory is the mean-variance portfolio selection approach of Markowitz (1952, 1959). We discuss the role of factor models in implementing portfolio selection, defining the nature of systematic risk, and estimating the premium for risk bearing

Factor models of security returns decompose the random return on each of a cross-section of assets into pervasive components, affecting almost all assets, and a diversifiable component. We describe four alternative approaches to factor models of asset returns. We also discuss issues related to...

An important issue in applications of multifactor models of asset returns is the appropriate number of factors. Most extant tests for the number of factors are valid only for strict factor models, in which diversifiable returns are uncorrelated across assets. In this paper we develop a test...