International Journal of Epidemiology, Vol 28, 319-326, Copyright © 1999 by International Epidemiological Association
RJ Hayes and S Bennett
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. CASE STUDIES:
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.
ARTICLES
Simple sample size calculation for cluster-randomized trials
MRC Tropical Epidemiology Group, London School of Hygiene and Tropical Medicine, UK.
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