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