An arm is a group or subgroup of participants in clinical trials. The group may receive a specific intervention or no intervention, in accordance with the trial’s protocol.

Several types of arms may be used in trials

  • Experimental arm: Group that receives the intervention that is the focus of the investigation.
  • Active comparator arm: Group receives another therapy to treat the same condition as the experimental therapy that is considered to be effective.
  • Placebo comparator arm: Group receives an inactive therapy.
  • Sham comparator arm: Group received an inactive therapy that is made to look identical to the intervention being studied.
  • No intervention arm.

Most clinical trials have multiple treatment arms, however, Phase I and II trials may involve single-arm trials in which the enrolled participants are all given the experimental therapy and followed.

Source: NIH. (n.d.). NCBI -Glossary of Common Site Terms. ClinicalTrials.Gov. Retrieved January 26, 2022, from