Community Mental Health Centre Proximity and Unmet Psychosis Treatment Among Nigerian Adults: A Geographic Regression-Discontinuity Estimation of Distance-Decay Exposure

Nigeria's community mental health centres are unevenly distributed, and the extent to which geographic proximity to these facilities shapes untreated psychosis remains poorly quantified. Because no individual-level treatment data are available for this analysis, the paper is methodological: it formalises distance-decay exposure as a continuous running variable, derives the discontinuity estimator at catchment thresholds, and sets out the falsification and sensitivity checks that a field implementation would require. The design is calibrated against the DHS Program indicator 'Knowledge of any method of contraception among married women' (FP_KMET_W_ANY), which provides 162 country-survey observations across 38 African countries from 1986 to 2025 (mean 91.776, standard deviation 11.342, range 57.80 in Chad to 99.90 in Eswatini). That indicator is used only to illustrate the distributional and cross-country heterogeneity that a proximity design must accommodate; it is not a psychosis outcome.