How you split data determines whether your evaluation metrics are trustworthy. Leakage, class imbalance, and temporal ordering each demand different splitting strategies, and getting them wrong silently inflates your scores.
Undersample for Class Balance
~15 min· Easy
Walk-Forward Cross-Validation Splits
K-Fold Size Allocation
~10 min· Intro
Stratified Sample Allocation
~20 min· Medium
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Optional multiple-choice questions on the ideas behind this section. Most useful after you have tried the coding problems above.