Simon Roth

Simon Roth

Data leakage affected 294 published papers across 17 scientific fields (Kapoor & Narayanan, 2023). The dominant response has been documentation: checklists, linters, best-practice guides. Documentation does not prevent these failures. This paper proposes a structural remedy: a grammar that decomposes the supervised learning lifecycle into 7 kernel primitives connected by a typed directed acyclic graph (DAG), with four hard constraints that reject the two most damaging leakage classes at call time. The grammar's core contribution is the terminal assess constraint: a runtime-enforced evaluate/assess boundary where repeated test-set assessment is rejected by a guard on a nominally distinct Evidence type. A companion study across 2,047 experimental instances quantifies why this matters: selection leakage inflates performance by d_z = 0.93 and memorization leakage by d_z = 0.53-1.11. Three separate implementations (Python, R, and Julia) confirm the claims. The appendix specification lets anyone build a conforming version.

Simon Roth

Twenty-eight within-subject counterfactual experiments across 2,047 tabular datasets, plus a boundary experiment on 129 temporal datasets, measuring the severity of four data leakage classes in machine learning. Class I (estimation - fitting scalers on full data) is negligible: all nine conditions produce $|Δ\text{AUC}| \leq 0.005$. Class II (selection - peeking, seed cherry-picking) is substantial: ~90% of the measured effect is noise exploitation that inflates reported scores. Class III (memorization) scales with model capacity: d_z = 0.37 (Naive Bayes) to 1.11 (Decision Tree). Class IV (boundary) is invisible under random CV. The textbook emphasis is inverted: normalization leakage matters least; selection leakage at practical dataset sizes matters most.