This label covers statistical learning outside deep networks: random forests and gradient boosting for cross-sectional return prediction, LASSO and ridge for factor selection, kernel methods and Gaussian processes for nonlinear signals, clustering for regime and asset grouping, and the model-selection machinery around them. In asset pricing it is the empirical workhorse of the “machine learning in the cross-section” literature.

What to check when reading. Financial data breaks the i.i.d. assumption behind ordinary cross-validation. A credible paper uses purged, embargoed, or walk-forward splits; reports feature importance with some stability check; shows the economic result (long-short portfolio after costs, not R-squared alone); and tests whether gains survive the removal of micro-caps and the inclusion of transaction costs. Out-of-sample R-squared in the cross-section is small when honest; a paper reporting 10% should be read with care.