Use of simulation with ipdpower in designing a randomised cluster study of an oral health intervention in care homes

This paper illustrates the use of a recently developed Stata procedure ipdpower (Kontopantelis, E.) in designing a cluster randomised trial. The trial required to compare change pre and post between intervention and non-intervention care homes. Forty nine residential care homes ranging in size from 3 to 112 beds (median 27 beds) were available to take part. Primary outcome measures were tooth cleaning (a dichotomy) and the Geriatric Oral Health Assessment Index (GOHAI, a continuous score). As is common in this situation it was required to explore the effect on sample size and power of a range of values of cluster sizes, within cluster correlation, between group variation, and intraclass correlation. Ranges of parameter values for a number of runs of the simulation procedure were obtained from published results of studies with similar features, transformed where necessary through standard formulae. The final design resulted in a recommendation of use of 16 homes with estimated statistical power of 80% for comparison of intervention with non-intervention participants, adjusting for baseline values. Simulation can be recommended as a valuable approach since it takes account of all features of the design, it facilitates communication among members of the study team in balancing design features and it provides a clear sense of the size required for the necessary statistical power.