You own a credit-card fraud classifier deployed as a probability scorer. Choose an operating threshold under asymmetric costs and justify it quantitatively. Assume per 1,000,000 transactions: base fraud rate = 0.20%. Costs: False Positive (decline a good transaction) = $15; False Negative (missed fraud) = $100. Consider three candidate thresholds with the following operating points on a representative validation set: - T1: TPR = 0.90, FPR = 0.020 - T2: TPR = 0.80, FPR = 0.010 - T3: TPR = 0.65, FPR = 0.004 Tasks: - For each threshold, compute the expected number of TP, FP, FN, TN and the expected total cost. Pick the threshold that minimizes expected cost and explain the business trade-offs. - Explain how you would calibrate scores (e.g., Platt/Isotonic), monitor for dataset/label drift, and periodically re-tune the threshold by segment (country, merchant category, transaction amount). - Propose guardrail metrics to detect harmful side effects (e.g., surge in declines for high-LTV users) and an experiment to validate changes without causing unacceptable customer pain.