The split-plot design was useful for evaluating complex, multilevel interventions, but there is need for improvement in its design and report

Abstract

Objectives: To describe the sample size calculation, analysis and reporting of split-plot (S-P) randomized controlled trials in health care (trials that use two units of randomization: one at a cluster-level and one at a level lower than the cluster).

Study design and setting: We carried out a comprehensive search in the EMBASE database from 1946 to 2016. Health care trials with a S-P design in human subjects were included. Three authors screened and assessed the studies, and the data were extracted on methodology and reporting standards based on CONSORT.

Results: Eighteen S-P studies were included, with authors using nine different designations to describe them. Units of randomization were unclear in nine abstracts. Explicit rationale for choosing the design was not given. Ten studies presented a sample size calculation accounting for clustering; the analyses were coherent with that. Flow of participant diagrams was presented but was incomplete in 14 articles.

Conclusion: S-P designs can be useful complex designs but challenging to report. Researchers need to clearly describe the rationale, sample size calculation, and participant flow. We provide a suggested CONSORT style participant flow diagram to aid reporting. There is need for more research regarding sample size calculation for S-P.

Keywords: 2x2; Cluster randomised; Factorial; Randomised controlled trial; Split-plots; Systematic review.