Cross-validation estimates how well a trained predictive model is likely to perform on new data by repeatedly separating available data into training and validation portions. Its reassurance depends on a key condition: future data must behave similarly to past data. It answers whether a prediction system is likely to work as expected, not whether changing the system’s inputs will cause a desired outcome. section 1.2
Use cross-validation to compare alternative supervised-learning models and select the one with stronger predictive capability on held-out test data. For a classifier that supplies probabilities, such as a propensity-score model, evaluate discrimination with AUC rather than accuracy when the target is the probability of treatment. section 5.3.2 section 5.3.2
Accurate predictions can still mislead interventions
A game-level dataset makes bonuses decrease as ghosts increase, while both affect completion time. A regression using bonuses alone has an R-squared of 0.9969; adding ghosts raises it only to 0.9996, so both appear similarly predictive. But when a designer changes the historical relationship and adds ghosts and bonuses together, the models make sharply different predictions. Cross-validation could assess predictive error under historical-like data, yet it cannot establish which model correctly predicts the consequence of a new design. section 4.1.5
Keep the boundary between prediction and causal inference explicit:
- Treat a good cross-validation result as evidence for prediction when the deployment setting resembles the observed data-generating setting. section 1.2
- Do not select a causal model solely by cross-validation, because predictive fit does not determine the effect of an intervention. section 4.6
- When machine learning supplies conditional probabilities for adjustment with many confounders, use cross-fitting even if cross-validation selected the predictive model. section 4.6
- Interpret high AUC differently for propensity scores: near-perfect treatment prediction can indicate poor comparability between treated and untreated cases. section 5.3.2











