International Journal of Epidemiology 2003;32:755-762
© International Epidemiological Association 2003
Special Theme: Infectious Diseases |
The effects of alternative study designs on the power of community randomized trials: evidence from three studies of human immunodeficiency virus prevention in East Africa
1 London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK.
2 Department of Public Health, Institute of Health Sciences, University of Oxford, Old Road, Oxford OX3 7LF, UK.
3 Johns Hopkins University, Bloomberg School of Public Health, 615 N Wolfe Street, Baltimore, MD 21205, USA.
4 MRC Programme on AIDS in Uganda, PO Box 49, Entebbe, Uganda.
Jim Todd, ITD, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK. E-mail: jim.todd{at}lshtm.ac.uk
Background Randomized intervention trials in which the community is the unit of randomization are increasingly being used to evaluate the impact of public health interventions. In the design of community randomized trials (CRT), the power of the study is likely to be affected by two issues: the matching or stratification of communities, and the number and size of the communities to be randomized.
Methods Data from three East African community intervention trials, designed to evaluate the impact of interventions to reduce human immunodeficiency virus (HIV) incidence, are used to compare the efficiency of different trial designs.
Results Compared with an unmatched design, stratification reduced the between-community variation in the Mwanza trial (from 0.51 to 0.24) and in the Masaka trial (from 0.38 to 0.28). The reduction was smaller in the Rakai trial where the selected communities were more homogeneous (from 0.15 to 0.11). For all trials, individual matching of communities produced estimates of between-community variation similar to those from the stratified designs. The linear association between HIV prevalence and incidence was strong in the Mwanza trial (correlation coefficient R = 0.83) and the Masaka trial (R = 0.83), but weak in the Rakai trial (R = 0.28). Unmatched study designs that use smaller communities tend to increase between-community variation, but reduce the design effect and improve study power.
Conclusions These empirical data suggest that selection of homogeneous communities, or stratification of communities prior to randomization, may improve the power of CRT.
Keywords Intervention studies, community-randomized, between-community variance, sample size, stratification, matching, power, study design, HIV
Accepted 12 February 2003
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