What is the main purpose of a crossover trial design?

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In this design, each participant receives multiple interventions in a random sequence and acts as their own control. This within-subject comparison reduces variability from differences between people, so you can detect treatment differences with greater precision and often with a smaller sample size. The approach works best when the condition is stable over time and treatment effects are reversible, allowing responses in one period not to bias subsequent periods. A washout period between treatments helps prevent carryover effects from one treatment period to the next. Because comparisons are made within the same individuals, you obtain a direct estimate of which treatment works better for each person. It isn’t about enrolling more participants, and randomizing the order remains important to avoid sequence effects. Placebo use isn’t the defining feature, though it can be included; the main goal is the efficiency and within-subject comparison. Crossover designs are less suitable when the disease progresses over time or when lasting treatment effects could confound later periods.