Supervised toothbrushing programmes delivered by dental hygiene therapists and caries increment in Tanzanian preschools: a cluster-randomised implementation trial of fidelity thresholds

Supervised toothbrushing programmes delivered by dental hygiene therapists are widely promoted for caries prevention in African preschools, yet their effectiveness depends on implementation fidelity that is rarely quantified before scale-up. The analysis characterises the operating characteristics of the BACI estimator across cluster counts of 8, 16, 24 and 32 and effect sizes of 3, 5 and 8 percentage points, with 2000 replications per design cell. Under a between-cluster standard deviation of 6.0, a residual standard deviation of 4.5 and an intraclass correlation of 0.64, power was estimated for each design cell. The results indicate that 52 clusters are required to detect a 5-percentage-point effect at 80% power, and that the minimum detectable effect declines steeply as cluster count rises from 8 to 32. The intraclass correlation of 0.64 substantially inflates the sample size required relative to individually randomised designs. Fidelity thresholds in supervised toothbrushing trials must be specified in advance and that the BACI design, when powered against realistic intraclass correlation values, provides a defensible framework for implementation trials in Tanzanian preschools. The contribution is a design template that links fidelity thresholds to statistical power, enabling dental hygiene therapists and programme managers to plan cluster-randomised evaluations that are both feasible and informative.