Trisha Shetty (Editor)

Stepped wedge trial

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A stepped-wedge trial is a form of randomised controlled trial that involves sequential but random rollout of an intervention over multiple time periods. The term "stepped wedge" was coined by the Gambia Hepatitis Intervention Study due to the stepped-wedge shape that is apparent from a schematic illustration of the design. The crossover is in one direction, typically from control to intervention, with the intervention not removed once implemented. The stepped-wedge design can be used for individually randomised trials, but is more commonly used as a cluster randomised trial (CRT).

In the context of a cluster randomised trial, the stepped-wedge design involves the collection of observations at a baseline period in which no clusters are exposed to the intervention. Following this, at regular intervals, or steps, a cluster (or group of clusters) is randomised to receive the intervention. This process continues until all clusters have crossed over to receive the intervention. Observations are taken at every cluster and at each time period. Stepped-wedge studies typically have one time period in which observations are made while all clusters are unexposed to the intervention, and one time period in which all clusters are exposed to the intervention.

In a stepped-wedge CRT, more clusters are exposed to the intervention at later than earlier time periods. As such, it is possible that an underlying temporal trend may confound the intervention effect, and so the confounding effect of time must be accounted for in both pre-trial power calculations and post-trial analysis

The design and analysis of stepped-wedge trials is therefore more complex than for other types of randomised trials. Previous systematic reviews highlighted the poor reporting of sample size calculations and a lack of consistency in the analysis of such trials. Hussey and Hughes were the first authors to suggest a structure and formula for estimating power in stepped-wedge studies in which data was collected at each and every step. This has now been expanded for designs in which observations are not made at each step as well as multiple layers of clustering. Additionally, a design effect (used to inflate the sample size of an individually randomised trial to that required in a cluster trial) has been established, which has shown that the stepped wedge CRT could reduce the number of patients required in the trial compared to other designs.

SPECIAL ISSUES AND CONFERENCES

The number of studies using the design have been on the increase. In 2015 a thematic series was published in Trials, Torgerson (2015). In 2016 the first international conference dedicated to the topic was held at the University of York. Proceedings from the conference were also published in Trials edited by Kanaan et al. (2016)

References

Stepped-wedge trial Wikipedia