PSI for Fraud Model Monitoring

Published: Jul 2026

Population Stability Index (PSI) is a simple way to measure score distribution drift between a reference period and a current period.

PSI Formula

For score bins \(i = 1,2,\dots,n\):

where \(E_i\) is the expected (reference) proportion and \(A_i\) is the actual (current) proportion.

Interpretation

• \(\text{PSI} < 0.1\): low drift
• \(0.1 \le \text{PSI} < 0.25\): moderate drift
• \(\text{PSI} \ge 0.25\): significant drift

PSI does not replace performance metrics, but it is a strong early warning signal for model and data pipeline shifts.