Key features of clinical trial design - crossover and factorial design

Definitions

Crossover trials have each participant receive multiple interventions in sequence (acting as their own control), reducing between-subject variability and requiring smaller sample sizes than parallel-group designs, but are only appropriate for conditions that are stable/chronic (not curable or rapidly progressive) and treatments without a prolonged carryover effect; factorial designs test two or more interventions simultaneously in combination, efficiently answering multiple research questions within a single trial
Used in specific circumstances where their particular design advantages are applicable, rather than as a default trial design choice given their more restrictive appropriateness criteria compared with standard parallel-group randomised trials

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