Simple sample size calculation for cluster-randomized trials.
BACKGROUND
Cluster-randomized trials, in which health interventions are allocated randomly to intact clusters or communities rather than to individual subjects, are increasingly being used to evaluate disease control strategies both in industrialized and in developing countries. Sample size computations for such trials need to take into account between-cluster variation, but field epidemiologists find it difficult to obtain simple guidance on such procedures.


METHODS
In this paper, we provide simple formulae for sample size determination for both unmatched and pair-matched trials. Outcomes considered include rates per person-year, proportions and means. For simplicity, formulae are expressed in terms of the coefficient of variation (SD/mean) of cluster rates, proportions or means. Guidance is also given on the estimation of this value, with or without the use of prior data on between-cluster variation.


METHODS
The methods are illustrated using two case studies: an unmatched trial of the impact of impregnated bednets on child mortality in Kenya, and a pair-matched trial of improved sexually-transmitted disease (STD) treatment services for HIV prevention in Tanzania.
Hayes
R J
RJ
MRC Tropical Epidemiology Group, London School of Hygiene and Tropical Medicine, UK.
Bennett
S
S
D002675:Child, Preschool; D016000:Cluster Analysis; D006801:Humans; D007630:Kenya; D016032:Randomized Controlled Trials as Topic; D018401:Sample Size; D012680:Sensitivity and Specificity; D013636:Tanzania; D006113:United Kingdom