Data Drift vs Concept Drift
Why models that hit 99% accuracy in offline testing decay silently 3 months after deployment.
Data Drift (Covariate Shift) occurs when feature input distributions P(X) change over time while target rules P(Y|X) remain fixed. Concept Drift occurs when the relationship between features and target P(Y|X) changes. Detecting drift requires statistical tests like Population Stability Index (PSI), Kolmogorov-Smirnov (KS) test, and adversarial drift classifiers. Mitigations range from feature re-scaling to retraining schedules.