RCT Variants
Core summary
Beyond the standard parallel-group RCT, several variants exist for specific clinical situations. Crossover trials test both treatments in the same patient. Cluster trials randomize groups, not individuals. Non-inferiority trials test whether a new treatment is 'not worse' rather than 'better.'
Detailed explanation
Detailed explanation
Parallel-group RCTs assign each participant to one group for the entire study — this is the most common design. Crossover RCTs assign each participant to both treatments sequentially, with a washout period between them. Each patient serves as their own control, reducing variability and requiring fewer participants. However, carryover effects (the first treatment still working during the second period) can bias results. Factorial designs test two or more interventions simultaneously in a 2x2 (or larger) grid, allowing assessment of interactions. Cluster RCTs randomize entire groups (hospitals, clinics, classrooms) rather than individuals — used when individual randomization is impractical or when the intervention is delivered at the group level (e.g., a hospital-wide protocol). Superiority trials aim to show one treatment is better. Non-inferiority trials aim to show a new treatment is 'not worse than' the standard by more than a pre-specified margin. Equivalence trials aim to show two treatments produce similar results within a defined margin.
Clinical example
You want to compare two inhalers for asthma. Since asthma is chronic and individual responses vary greatly, a crossover design works well: each patient uses Inhaler A for 4 weeks, washes out for 2 weeks, then uses Inhaler B for 4 weeks. You compare their lung function on each inhaler.
Research example
The ISIS-2 trial used a 2×2 factorial design to simultaneously test aspirin and streptokinase in acute myocardial infarction. This elegant design showed that both treatments reduced mortality, and that the combination was even more effective — answering two questions in one trial.
Knowledge check
Q1. What is the main advantage of a crossover trial?
Q2. When is a non-inferiority trial appropriate?
Q3. Why would you use cluster randomization instead of individual randomization?