What is a 2x2 crossover design?

In a typical 2x2 crossover study, participants in two groups each receive a test drug and a reference drug. Measuring the effects of both drugs in the same participants allows you to reduce the amount of variability that is caused by differences between participants.

Each group receives the drugs in a different sequence. For example, in the illustration below, participants for Sequence 1 receive the reference drug during Period 1, followed by the test drug during Period 2. Participants in Sequence 2 receive the test drug during Period 1 and the reference drug during Period 2.

A washout period between Period 1 and Period 2 allows the effects of one drug to dissipate before the next drug is administered.