Describes a clinical study in which groups of participants receive one of several combinations of interventions. For example, a two-by-two factorial design involves four groups of participants. Each group receives one of the following pairs of interventions: 1) drug A and drug B, 2) drug A and a placebo, 3) a placebo and drug B, or 4) a placebo and a placebo. So during the trial, all possible combinations of the two drugs (A and B) and placebos are given to different groups of participants. One type of Intervention model (design).