C = number of matched clusters Zα/2 = level of significance, 95% = 1.96 Zβ = power of the study, 80% = 0.84 X0 = the estimated average annual smear-positive case detection rate in the control sites = 40% X1 = the estimated average annual smear-positive case detection rate in the intervention sites = 60% K= coefficient of variation = 0.25 Av (1/X0j) = the mean of the reciprocals of the cluster sizes (person-years) in the control group = 0.001 Av (1/X1j) = the mean of the reciprocals of the cluster sizes (person-years) in the intervention group = 0.0014 This computation provides a cluster size of 8.6 pairs. Considering feasibility, the investigators chose 8 pairs.