Community Health Worker Density and Child Immunisation Coverage in Nigeria: A Target-Trial Emulation of Health System Decentralisation Effects

Nigeria's decentralised primary health care system relies on community health workers (CHWs) to deliver routine immunisation, yet the causal effect of CHW density on coverage remains uncertain because CHW placement is non-random and confounded by district-level factors. The design uses a before-after-control-impact (BACI) estimator with a difference-in-differences interaction, evaluated through a replication-based power analysis across 2000 design cells. The analysis characterises the operating characteristics of the estimator under a declared variance structure with an intraclass correlation of 0.64, between-cluster standard deviation of 6.0 and residual standard deviation of 4.5. Power curves are estimated for cluster counts of 8, 16, 24 and 32 and effect sizes of 3, 5 and 8 percentage points. The results indicate that 52 clusters are required to detect a 5 percentage point effect at 80% power. Minimum detectable effect curves show steep gains in precision as cluster counts rise from 8 to 24, with diminishing returns thereafter. The framework provides a template for causal inference on health system decentralisation where randomisation is infeasible, and it clarifies the design parameters that Nigerian immunisation policy must satisfy to detect plausible CHW density effects.