Evidence-appraisal glossary
External control arm
An external control arm is a comparison group drawn from outside the current study, such as past trials, registries, or health records, rather than from people randomized alongside the treated group. It lets researchers estimate a treatment's effect when a concurrent randomized control group is impractical, but it carries a higher risk of biased comparisons.
Also called: synthetic control arm, historical control, external comparator.
An external control arm (also called a synthetic or historical control) supplies the comparison group for a study from data collected outside it, such as earlier trials, disease registries, or electronic health records, instead of from participants randomly assigned at the same time. It is used when a randomized concurrent control is difficult, for instance in rare diseases or single-arm trials of promising therapies. The main weakness is confounding: because patients were not randomized, differences in era, eligibility, supportive care, or how outcomes were measured can masquerade as treatment effects. When reading such a study, ask how comparable the external group is to the treated patients, whether the analysis adjusted or matched on key characteristics, and whether outcomes were defined and measured the same way in both sources. For example, a single-arm trial of a new cancer drug might compare survival against a registry of similar patients treated earlier. Treat the resulting effect estimate more cautiously than one from a randomized comparison.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.