Adaptive Clinical Trial Designs

Definition & core concept

• Definition — An adaptive clinical trial design allows pre-planned modifications to one or more aspects of the design (sample size, allocation, treatment arms, study population, endpoints) based on interim analyses of the accumulating data, while preserving statistical validity and integrity. FDA: "a clinical trial that allows for prospectively planned modifications to one or more aspects of the design based on accumulating data from participants." [FDA]
• Two operative phrases — Prospectively planned and based on accumulating data — the adaptation rules are specified in the protocol/SAP before the trial begins and before any data are examined; adaptation is "by design," not a reactive post-hoc amendment.
• Contrast with the fixed trial — The classic ("fixed") trial runs to a protocol that does not change during the study — sample size, allocation ratio, doses and analysis are all fixed in advance. (Fixed parallel/crossover/factorial designs are out of scope — contrasted only as comparators.)
• Feasibility constraint — Adaptation is feasible only when outcomes are measured early enough to allow design changes while recruitment continues — a short-term continuous/surrogate endpoint permits it; a long-latency outcome (e.g. 5-year dementia incidence) leaves no window. Adaptation is information-driven, not calendar-driven (triggered by an accrued "information fraction").
• Two governing goals — Every adaptive design must simultaneously (i) preserve the overall Type-I error probability (conventionally ≤ 0.05 two-sided) and (ii) yield treatment-effect estimates with minimal bias — both are threatened by the act of looking at and acting on interim data.