A crossover study design is also called a repeated measurements design, where experimental units receive all the treatments in different periods.

For instance, consider a clinical trial comparing drug A and drug B on 10 asthmatic patients randomly divided into group one, and group two.

First, each group receives different drugs for two weeks, recording their effect on the patient's physiology. This is followed by a washout period, to eliminate the drug from the patient's body.

Now, the groups are switched so that the second group receives drug A, and the first group receives drug B. This is termed a cross-over design. In this design, subjects act as their own controls, and their characteristics are not changed throughout the study. It also removes inter-subject variabilities.

This design is generally used in late-phase clinical trials involving drugs that help control the symptoms and not cure them completely.

For instance, in the previous example, drug B will not have an opportunity to demonstrate its effectiveness if drug A cures the patient during the first period.