Mounting a community-randomized trial: sample size, matching, selection, and randomization issues in PRISM
Section snippets
Background
This paper discusses some of the processes for establishing a cluster-randomized trial of a community and primary care intervention in 16 local government areas (LGAs) in Victoria, Australia. It traces the development of the trial in terms of design factors such as sample size estimates and the selection and randomization of communities to intervention or comparison prior to trial commencement. The protocol for the trial is presented and discussed elsewhere [1].
In brief, prior population-based
Number of clusters and matching in pairs
The intervention program to be implemented in PRISM was conceived as a whole community approach to improving support for all mothers in the first 12 months after birth. A cluster-randomized trial was thus the design of choice from the outset. In the Victorian context, LGAs were chosen as the appropriate cluster unit because of their statutory responsibility for community service provision to mothers and babies post-hospital discharge and for their data systems based on birth notifications from
Sample size estimates
Sample size estimates where the intervention is assigned at the individual level depend on the primary health outcomes, a baseline or comparison group value, and a minimum program effect to be reliably detected at 80% power.
The primary health outcomes for mothers were prevalence of maternal depression determined by a score of 13 or more on the Edinburgh Postnatal Depression Scale (EPDS) [21] and maternal physical health functioning and mental health summary scores of the health status measure,
Estimates of components of variance
There are two components of variance: the assumed binomial variation within clusters (mothers' depression) (σw2) and a between-cluster (normal) variation (σc2), which comprise within-cluster and between-pair variation, respectively. The proportion of cluster variation reduced by the matching is the correlation ρM. The usual intraclass correlation is also reduced by ρM (see Appendix A).
The effect on sample size then depends on the interplay of assumptions about σc2 and ρM with σw2 estimated as
Missingness
The impact of missingness was considered in two ways: missingness due to nonresponse from women within the communities and missingness due to the possible loss of a cluster. Nonresponse was estimated to be around 33% [27]; this was allowed for by inflating the numbers of births required by 50%. Since numbers required corresponded to the average annual number of births in the communities, the data collection time was increased to 18 months in the modified sample size calculations of 1998. By
Criteria for determining possible matched pairs and potential sets of LGAs : defining the final set of participating LGAs
Pairing between communities was undertaken to minimize potential imbalance between the comparison and intervention arms of the study in the baseline level of the primary outcomes and in associated risk factors, such as the size of each community, the size of the population of interest, and community capacity to implement the intervention.
Reliable maternal depression prevalence data by LGA were not available, so a range of data was obtained to assist with determining useful matching criteria,
Minimizing potential contamination between LGAs
A grid was used to assess possible matches within metropolitan and rural strata. The factors on the margins were data available at a community level (number of births, number of non-English-speaking background mothers, area, population size, estimates of population change, community infrastructure rating, and geographical north/south and east/west grid references). By sorting rows and columns on any of these factors, close matches could be identified and, from the map grids, close communities
Selection of LGAs
With communities enrolled, possible pairs determined, and contamination issues assessed, a two-stage process for selection of the communities was employed to assign the intervention using an appropriate randomization process, designed to promote goodwill and continuing cooperation of all groups involved.
Stage 1 entailed the selection of one final LGA set from all possible sets of metropolitan and rural pairs. Only one metropolitan set of four independent pairs was possible resulting in all
Discussion
At the time this study was first designed, relatively few studies in which the logical or economic unit of randomization was the group or community had been published, and of those, some had quite severe design limitations. Since then, the interest in such studies has burgeoned so that not only are there many published design and results papers available [31], [32], [33], [34], [35], there exist commentaries [20], [36], [37], cautionary tales [38], [39], and texts devoted to the methodology
Acknowledgements
This study was funded from a range of sources: La Trobe University and Victorian Department of Human Services Collaborative Industry grant 1997, National Health and Medical Research Council project grants 1997–1999 and 1999–2001, 2002–2004, Victorian Department of Human Services program implementation and evaluation grants 1998–2000, Victorian Health Promotion Foundation grants for program resources and implementation 1998–2000, Felton Bequest grant 1998, Sidney Myer Fund grant 1999, and
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