Overview
Many quantitative equity portofolio models create return and risk forecasts using a set of statistical factors that drive the equity returns. The theory establishing such much is given in Arbitrage Pricing Theory
Factors
Equity factors are typically classified into the following three categories.
The Fama/French is a famous three factor model whose effectiveness has seemed to persist over time.
Fitting
Once the risk factors have been identified, one needs to estimate an assets sensitivity (beta) to the factor in order to use the model in practice. This involves fitting the model to the available data. There are multiple techniques for fitting a model to data.