12.6:
Crossover Experiments
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.
12.6:
Crossover Experiments
Crossover experiments, also called the repeated-measurements design, is a study design in which all experimental units are exposed to all treatments in different periods. Crossover experiments are generally used in psychology, the pharmaceutical industry, agriculture, and medicine.
Crossover designs are performed even with smaller sample sizes since the samples can act as their controls. These are better than simple randomized trials since patients are exposed to all the treatments. Furthermore, cross-over designs are preferred to compare the bioavailability or ingestion of the drug in the human body and compare it with a reference drug. This method is unsuitable if the disease is chronic and stable and the drug should not completely cure the disease condition.