School-based fluoride varnish programmes and caries incidence in Kenyan primary pupils: a target-trial emulation of hygiene-therapy delivery intensity

School-based fluoride varnish programmes are widely promoted as a caries-preventive strategy in low-resource settings, yet the intensity of hygiene-therapist delivery that programmes can feasibly sustain is rarely specified in advance. No observational data were used; the analysis characterises the operating characteristics of the design under a declared variance structure with a between-cluster standard deviation of 6.0, a residual standard deviation of 4.5 and an intraclass correlation of 0.640. Power was estimated across 2000 replications per design cell for 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 effect at 80% power. The results indicate that the before-after-control-impact estimator is sensitive to cluster count and effect magnitude, and that the intraclass correlation of 0.640 imposes a substantial design penalty. Target-trial emulation of hygiene-therapy delivery intensity is feasible only when cluster counts are planned against the intraclass correlation rather than against pupil-level sample size, and it specifies the boundary conditions under which the design can support causal claims about programme intensity.