Caption: Crossover Trial
Text: In a crossover trial participants are randomized first to one and then to another treatment so that each subject receives both of the treatments being compared. For example, a trial was conducted in patients with asthma to compare laser acupuncture with a sham procedure. Patients were randomized either to acupuncture for five weeks (first period) then sham for five weeks (second period), or sham then acupuncture. The primary outcome was a symptom score.

Since each subject received both treatments, one can compute a difference score for each subject, in essence quantifying the difference in response within each subject.

This is a very efficient design, because one does not have to control for all the variation that occurs between patients, which adds to the variability observed in a study with independent samples, i.e., unpaired.