In epidemiology, cross-over studies or crossover trials are a type of randomized controlled trial. Cross-over studies are "studies comparing two or more treatments or interventions in which the subjects or patients, upon completion of the course of one treatment, are switched to another. In the case of two treatments, A and B, half the subjects are randomly allocated to receive these in the order A, B and half to receive them in the order B, A. A criticism of this design is that effects of the first treatment may carry over into the period when the second is given."
Special issues in the reporting of cross-over trials are:
- Sequence generation
- Washout period
- Testing for carry-over effect
- Testing for period effect
- Testing for treatment effect
- Adjustment for period effect and maybe for carry-over effect
- Paired analysis
- Patient preference regarding intervention
"If carry over is detected convention suggests this may be dealt with in the analysis in one of two ways. The usual approach is to treat the study as though it were a parallel group trial and confine analysis to the first period alone."
The results of crossover studies "tend to agree with those of parallel arm trials, although there was a trend for more conservative treatment effect estimates in parallel arm trials".
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