- A type of trial design in which each subject receives more than one of the interventions under investigation in random order. It is different from a parallel groups design where some subjects receive one treatment and different subjects receive another. The crossover design represents a special situation where there is no separate comparison group, each subject in effect serving as its own control. A common procedure for comparing interventions A and B, for example, would be for the subjects to be randomly allocated to receive them in either the order A, B or the order B, A, switching from one to the other when the course of treatment ends. It is most often used to study treatments for stable health problems.