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.
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.
Todd
Jim
J
London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK. jim.todd@lshtm.ac.uk
Carpenter
Lucy
L
Li
Xianbin
X
Nakiyingi
Jessica
J
Gray
Ron
R
Hayes
Richard
R
D003153:Community Health Services; D015658:HIV Infections / Q000453:epidemiology / Q000517:prevention & control*; D006801:Humans; D015994:Incidence; D015995:Prevalence; D015397:Program Evaluation / Q000379:methods; D011634:Public Health; D016032:Randomized Controlled Trials as Topic / Q000379:methods*; D012107:Research Design; D018401:Sample Size; D013223:Statistics as Topic; D013636:Tanzania / Q000453:epidemiology; D014454:Uganda / Q000453:epidemiology