Intracluster correlation coefficients and coefficients of variation for perinatal outcomes from five cluster-randomised controlled trials in low and middle-income countries: results and methodological implications

Christina Pagel, Audrey Prost, Sonia Lewycka, Sushmita Das, Tim Colbourn, Rajendra Mahapatra, Kishwar Azad, Anthony Costello, David Osrin, Christina Pagel, Audrey Prost, Sonia Lewycka, Sushmita Das, Tim Colbourn, Rajendra Mahapatra, Kishwar Azad, Anthony Costello, David Osrin

Abstract

Background: Public health interventions are increasingly evaluated using cluster-randomised trials in which groups rather than individuals are allocated randomly to treatment and control arms. Outcomes for individuals within the same cluster are often more correlated than outcomes for individuals in different clusters. This needs to be taken into account in sample size estimations for planned trials, but most estimates of intracluster correlation for perinatal health outcomes come from hospital-based studies and may therefore not reflect outcomes in the community. In this study we report estimates for perinatal health outcomes from community-based trials to help researchers plan future evaluations.

Methods: We estimated the intracluster correlation and the coefficient of variation for a range of outcomes using data from five community-based cluster randomised controlled trials in three low-income countries: India, Bangladesh and Malawi. We also performed a simulation exercise to investigate the impact of cluster size and number of clusters on the reliability of estimates of the coefficient of variation for rare outcomes.

Results: Estimates of intracluster correlation for mortality outcomes were lower than those for process outcomes, with narrower confidence intervals throughout for trials with larger numbers of clusters. Estimates of intracluster correlation for maternal mortality were particularly variable with large confidence intervals. Stratified randomisation had the effect of reducing estimates of intracluster correlation. The simulation exercise showed that estimates of intracluster correlation are much less reliable for rare outcomes such as maternal mortality. The size of the cluster had a greater impact than the number of clusters on the reliability of estimates for rare outcomes.

Conclusions: The breadth of intracluster correlation estimates reported here in terms of outcomes and contexts will help researchers plan future community-based public health interventions around maternal and newborn health. Our study confirms previous work finding that estimates of intracluster correlation are associated with the prevalence of the outcome of interest, the nature of the outcome of interest (mortality or behavioural) and the size and number of clusters. Estimates of intracluster correlation for maternal mortality need to be treated with caution and a range of estimates should be used in planning future trials.

Figures

Figure 1
Figure 1
Location, population size and duration of studies.
Figure 2
Figure 2
Simulated impact of number of live births per cluster on estimated coefficient of variation (k) for maternal and neonatal mortality. These simulations were run assuming that all birth outcomes were independent, i.e. the true coefficient of variation was set to zero. As the number of births per cluster increases, the estimated coefficient of variation falls closer to zero indicating that more births per cluster lead to greater reliability in estimates of the coefficient of variation. Throughout, the estimated coefficient of variation is higher for maternal mortality than for neonatal mortality.
Figure 3
Figure 3
Simulated impact of number of clusters on estimated coefficient of variation (k) for maternal and neonatal mortality. These simulations were run assuming that all birth outcomes were independent, i.e. the true coefficient of variation was set to zero. As the number of clusters increases, the estimated coefficient of variation falls closer to zero indicating that more clusters lead to greater reliability in estimates of the coefficient of variation. Throughout, the estimated coefficient of variation is higher for maternal mortality than for neonatal mortality. In comparison with figure 2, the number of births per cluster has a greater impact on estimates of coefficient of variation than the number of clusters.

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