Trial Design
External Control Arm
An external control arm is a comparator group constructed from data outside the current clinical trial; historical trial data, registry data, EHR data, or natural-history studies; used to contextualize results from a single-arm trial when randomization is infeasible.
External controls are most defensible in rare diseases, oncology with high unmet need, and pediatric studies where withholding treatment is unethical. FDA's 2023 draft guidance Considerations for the Design and Conduct of Externally Controlled Trials for Drug and Biological Products formalizes acceptance criteria.
Methodological rigor requires pre-specified eligibility alignment, propensity-score or matching adjustment, sensitivity analyses, and transparency on data provenance.
Related terms
Real-World Evidence (RWE)
Real-World Evidence (RWE) is clinical evidence about the use, safety, and effectiveness of a medical product derived from analysis of Real-World Data (RWD); EHRs, claims, registries, wearables; collected outside the controlled environment of a randomized clinical trial.
Real-World Data (RWD) and Real-World Evidence (RWE)
Real-world data (RWD) is healthcare data collected outside the context of a controlled clinical trial; real-world evidence (RWE) is the clinical evidence about an intervention derived from analyzing RWD.
Randomization
Randomization is the process of assigning clinical trial participants to treatment groups by chance, eliminating selection bias and producing comparable groups for unbiased estimation of treatment effects.
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