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Cost-based threshold preview (on validation, never on test)

Choose the decision threshold that minimises 5·FN + FP using cross-validated predictions on train; report the test set once. Concepts: Decision threshold; cost curve; evaluation hygiene Project A: Your notebook tuned the threshold on the test set (best ≈ 0.35, cost 93) → redo the search on cross-validated train predictions; expect a similar but honest threshold. Project B: Not applicable. Equivalent: choose a forecast bias/quantile (e.g. P60 forecast) when under-forecasting costs more.