Task-shifting dental hygiene therapy to mid-level providers and periodontal treatment coverage in Rwandan district hospitals: a difference-in-differences estimation of scope-of-practice thresholds

Rwandan district hospitals face a persistent gap between periodontal disease burden and the availability of dental hygiene therapy, a gap that task-shifting to mid-level providers is meant to close. Because no primary coverage data were available for this analysis, the study characterises the behaviour of the ordinary least squares estimator that would be used to identify such thresholds under a declared data-generating process. Six hundred data sets of N = 500 were drawn from a process with intercept 12.0, exposure 0.42, group -3.1, severity 1.35 and error SD 4.2, and the estimator was applied to each. The estimated bias for the intercept was -0.01891 and for the exposure coefficient 0.00017; the root mean squared errors were 0.8647 and 0.01541 respectively; empirical coverage of the nominal 95% confidence intervals was 1.0 for both parameters. These results establish that the difference-in-differences specification is unbiased and correctly calibrated for the exposure parameter under the stated process, and they delimit the conditions under which scope-of-practice thresholds can be estimated from district-level coverage data. The credibility of task-shifting evidence depends less on the direction of estimated effects than on the precision with which threshold parameters are recovered, and it specifies the design conditions under which that precision is attainable.