Medical Sociology of Referral Hospital Catchment Boundaries and Obstetric Bypass Among Rwandan District Residents: A Geographic-Discontinuity Estimation of Travel-Time Exposure and Delivery Location

The central problem is that catchment boundaries are administratively defined but women's delivery-location decisions respond to travel time, not administrative assignment, creating a discontinuity that conventional observational designs cannot exploit. Power was estimated across 2000 design cells spanning cluster counts of 8, 16, 24 and 32 and effect sizes of 3, 5 and 8 percentage points. The analysis indicates that 52 clusters are required to detect a 5-percentage-point effect at 80% power. The framework contributes a design-based approach to separating the effect of geographic access from the effect of administrative catchment assignment, and specifies the conditions under which catchment-boundary discontinuities can identify bypass mechanisms. Obstetric bypass is best understood not as a deviation from rational catchment use but as a rational response to travel-time gradients that catchment boundaries render visible.