Key concepts in clinical epidemiology: Stepped wedge trials.

A stepped wedge trial evaluates an intervention that is implemented over a number of time periods according to a staggered timetable. Stepped wedge trials are usually cluster randomized, the intervention being delivered at some geographical, service or other cluster level. There is considerable variety in the design and conduct of stepped wedge trials in practice. The analysis of a stepped wedge trial often assumes that the effect of the intervention is maintained at a constant level once it has been implemented. It is important when estimating this effect to adjust for a period effect or underlying secular trend, since time is confounded with intervention, and to account for the clustering of outcomes. The advantage often cited for a stepped wedge design is that every cluster ends up getting the intervention, though in any trial design we can offer the intervention preferentially to control clusters after the trial has finished. The real advantage of a stepped wedge design is likely to be practicality or statistical efficiency.