Validation, cross-validation, the bias–variance tradeoff, and pitfalls
Precision, recall, F1, ROC curves, and regression metrics
Splitting data to measure generalization and avoid overfitting
K-fold, stratified, and leave-one-out evaluation
Why models underfit or overfit — the fundamental decomposition
Diagnosing underfitting and overfitting from training size
SMOTE, class weights, and metrics beyond accuracy
Filter, wrapper, and embedded methods for choosing features
Why high-dimensional spaces break intuition and algorithms