A single-factor model of the economy classifies sources of uncertainty as systematic (macroeconomic) factors or firm-specific (microeconomic) factors. The index model assumes that the macro factor can be represented by a broad index of stock returns.

The single-index model drastically reduces the necessary inputs in the Markowitz portfolio selection procedure. It also aids in specialization of labor in security analysis.

According to the index model specification, the systematic risk of a portfolio or asset equals β²σ²_M and the covariance between two assets equals β_iβ_jσ²_M.

The index model is estimated by applying regression analysis to excess rates of return. The slope of the regression curve is the beta of an asset, whereas the intercept is the asset's alpha during the sample period. The regression line is also called the security characteristic line.

Optimal active portfolios constructed from the index model include analyzed securities in proportion to their information ratios. The full risky portfolio is a mixture of the active portfolio and the passive market-index portfolio. The index portfolio is used to enhance the diversification of the overall risky position.

Practitioners routinely estimate the index model using total rather than excess rates of return. This makes their estimate of alpha equal to α + r_f (1 — β).

Betas show a tendency to evolve toward 1 over time. Beta forecasting rules attempt to predict this drift. Moreover, other financial variables can be used to help forecast betas.