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In a planned cross-over design, all participants receive both study treatments (e.g., both Drug A and Drug B, or Drug A and placebo); after a trial period with the first therapy the patients are "crossed over" to the other therapy. Thus participants are given two or more treatments in sequence. As shown in this slide, participants are randomized to either Group I or Group II, which determines the order of the therapies - in this case, Group I will receive Drug A first, followed by Drug B, and Group II will receive Drug B first, followed by Drug A.

Because each patient serves as his/her own control, this design is more efficient than a non-crossover, parallel group type of design, thus allowing smaller sample size. However, because of the possibility that carry-over effects from the first to second period may be present (where effects of the first treatment may still be present for a long period, or where the first treatment permanently changes the course of the disease), cross-over trials cannot be used for certain types of treatment comparisons. Methods to minimize carry over effects and to test for the presence of carry-over are available.

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