Portfolio return may be evaluated using factor models. The first model, proposed by Jensen (1968), relies on the CAPM and explains portfolio returns with the market index as the only factor. It quickly becomes clear, however, that one factor is not enough to explain the returns very well and that other factors have to be considered. Multi-factor models were developed as an alternative to the CAPM, allowing a better description of portfolio risks and a more accurate evaluation of a portfolio's performance. For example, Fama and French (1993) have highlighted two important factors that characterize a company's risk in addition to market risk. These factors are the book-to-market ratio and the company's size as measured by its market capitalization. Fama and French therefore proposed three-factor model to describe portfolio normal returns (Fama–French three-factor model). Carhart (1997) proposed to add momentum as a fourth factor to allow the short-term persistence of returns to be taken into account. Also of interest for performance measurement is Sharpe's (1992) style analysis model, in which factors are style indices. This model allows a custom benchmark for each portfolio to be developed, using the linear combination of style indices that best replicate portfolio style allocation, and leads to an accurate evaluation of portfolio alpha.