2604.01163 The Stratification Instability Index: Propensity Score Subclassification Produces Unstable Treatment Effect Estimates Below 5 Strata
Propensity score subclassification partitions units into strata based on estimated propensity scores, then estimates treatment effects within each stratum. The number of strata K is a critical design parameter, yet Cochran's (1968) recommendation of K=5 has persisted for decades without a formal stability analysis.