Urban development-induced relocation of informal settlement clinics and tuberculosis-treatment continuity in Nairobi: a difference-in-differences estimation of clinic-catchment thresholds

Relocation of informal settlement clinics under urban development programmes is often justified by catchment rationalisation, yet the threshold at which a clinic's removal disrupts tuberculosis-treatment continuity remains unquantified. the present analysis treats the before-after-control-impact estimator as a design problem and characterises its operating characteristics under a declared variance structure for clinic-catchment tuberculosis outcomes. Using a replication-based evaluation with between-cluster standard deviation of 6.0, residual standard deviation of 4.5 and an intraclass correlation of 0.640, power was estimated across 2000 design cells defined by cluster counts of 8, 16, 24 and 32 and effect sizes of 3, 5 and 8 percentage points. The design required 54 clusters to detect a 5-percentage-point change at 80 per cent power. Estimated power rose with effect size and cluster count, and the minimum detectable effect at 80 per cent power fell as clusters increased. The results establish that catchment-threshold studies of clinic relocation are feasible only at cluster counts well above those typical of ward-level evaluations, and they specify the design conditions under which a relocation effect on treatment continuity can be separated from between-clinic heterogeneity.