StATS: What is a crossover design?

A research design where the subjects get both treatments in sequence. Contrast this with a parallel groups design where some subjects get the first treatment and different subjects get the second treatment. The crossover design represents a special situation where there is not a separate comparison group. In effect, each subject serves as his/her own control. Also, since the same subject receives both treatments, there is no possibility of covariate imbalance. Ideally in a crossover design, a subject is randomly assigned to a specific treatment order. Some subjects will receive the standard therapy first, followed by the new therapy (AB). Others will receive the new therapy first, followed by the standard therapy (BA). Here is an example of a crossover design:

• In Lokken et al (1995), patients received identical oral surgical procedures on two separate occasions. At one surgery, a patient received homeopathic treatment for pain. At the other surgery, a patient received placebo. The researchers measured postoperative pain, bleeding, swelling, and the reduction of the ability to open one's mouth.

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