Most large-scale clinical trials use a parallel experimental design in which randomly selected subjects are assigned to one of two or more treatment Arms . Once assigned to an Arm, each subject is given a single treatment, either the drug or drugs being tested, or the appropriate control (usually a placebo) for the duration of the study. Data are collected and subjected to between-patient analysis. Large sample sizes are usually required in these studies to account for the many sources of inter-subject variation, while still enabling accurate detection of any treatment effects.An increasingly popular strategy for clinical trials, particularly for those involving stable, chronic conditions, involves the use of a crossover design. In this design, every subject is sequentially given all of the treatments in the study. Each treatment is administered for a defined period of time. Each subsequent treatment is preceded by a recovery, or washout , period where no treatments are administered, to allow subjects’ conditions to return to their normal states. Patients are randomized only with respect to the order in which the different treatments are administered. Because each subject serves as his or her own control, reducing the effects of between-patient variation, and because each subject can be used multiple times, crossover studies usually require far few subjects, although for longer times, than comparable parallel studies. These benefits can often outweigh the risks (patient drop-out, changing patient condition over time, carryover of one treatment to the next due to insufficient washout, or any secondary effects), associated with this design.