A crossover study refers to a type of clinical trial in which groups of participants receive two or more interventions in a specific order. For example, two-by-two cross-over assignment involves two groups of participants. One group receives therapy A during the initial phase of the trial, followed by therapy B during a later phase. The other group receives therapy B during the initial phase, followed by therapy A. So during the trial, participants “cross over” to the other therapy. All participants receive therapy A and therapy B at some point during the trial but in a different order, depending on the group to which they are assigned.