Crossover trial
Short definition
A crossover trial is a trial in which each participant receives both the intervention and the comparison, in successive periods and in a randomly allocated order, acting as their own control.
In plain language
Because each person is compared with themselves, this design needs fewer participants. Between the two periods there is usually an interval without intervention (a washout period) so that the effect of the first does not contaminate the second. It suits stable conditions and short-lived effects; for example, comparing, in the same person, a virtual reality session and a music session on different days.