Crossovers

Two period (two-way), three period (three-way), four period or multi-period comparison (within- or between-subject); each subject receives two treatments one after the other (or simultanously e.g. left vs right for topical treatments), the order of treatment being decided randomly; although this is acommon design to demonstrate bioequivalence, clinical studies with such a design are infrequent (only for agents with prompt onset and rapid offset of effects and diseases with a stable course); → see design, heaton-ward effect.