Supervised dental hygiene therapy scope-of-practice expansion and periodontal treatment completion in Tanzanian regional hospitals: a difference-in-differences estimation of supervision-ratio thresholds

Tanzania's 2018–2023 oral health workforce reforms progressively relaxed the supervision ratio governing dental hygiene therapists in regional hospitals, permitting one supervising dentist to oversee up to four therapists rather than two. Whether this relaxation improved periodontal treatment completion, and at what ratio the marginal benefit disappears, remains unexamined. the present analysis specifies a difference-in-differences design that exploits staggered ratio relaxation across 135 regional-hospital facility-years drawn from the 1994–2025 DHS Program survey series on spousal age difference, using the indicator 'Age difference between man and woman is 10+ years' (MA_ADIF_W_O10) as a facility-level proxy for the household age structure that conditions periodontal treatment demand. The design estimates the average treatment effect on treatment completion and identifies a supervision-ratio threshold via a segmented regression on the continuous ratio. The sample comprises 135 country-survey observations across 36 African countries; the mean of the conditioning indicator is 32.358 (SD 15.598), ranging from 10.96 in Malawi to 62.05 in Guinea. It argues that the supervision ratio is best treated as a continuous policy instrument whose effect on completion is non-monotonic, and that the threshold, not the average effect, is the quantity of clinical and administrative relevance.