In the experimental study, the cross-over design is often used and it is a very powerful tool.  The cross-over study design is usually used in drug trials.  It is very effective if you do not have a large study cohort or participants must be on medication all the time.  Since individuals are serving themselves as a control, you do not need to find additional control subjects.  However, the research should have a clear knowledge about the washout period of the medication in trial.  If the washout period is not properly applied, the second medication will have a false and an efficacy.  This slide presents the single crossover design.  Double, triple designs are also available.