A clinical trial where groups of volunteers are administered two or more interventions in a specific order. For example, a “two-by-two” cross-over trial design is where one group receives drug A at the beginning of the trial and then receives drug B for the rest of the trial. In the second group, participants receive drug B first and then drug A. Thus, the term “cross-over” is used to describe the order in which they are assigned; for example, drug A and then drug B, or drug B and then drug A. All participants receive both drugs during the study.