Flowchart of four phases (enrollment, allocation, intervention, follow-up, and data analysis) of a parallel<unk>  trial of two groups (in a<unk>  trial, one of the interventions serves as the control), modified from the CONSORT (Consolidated Standards of Reporting Trials) 2010 Statement A<unk>  (or<unk>  control trial;<unk>  is a form of scientific experiment used to control factors not under direct experimental control. Examples of<unk> s are clinical trials that compare the effects of drugs, surgical techniques, medical devices, diagnostic procedures or other medical treatments. Participants who enroll in<unk> s differ from one another in known and unknown ways that can influence study outcomes, and yet cannot be directly<unk> . By randomly allocating participants among compared treatments, an<unk>  enables statistical control over these influences. Provided it is designed well, conducted properly, and enrolls enough participants, an<unk>  may achieve sufficient control over these confounding factors to deliver a useful comparison of the treatments studied.
Randomized controlled trial
Natural history study
Placebo-controlled study
Prospective cohort study
Multiple baseline design
Retrospective cohort study
Cohort study
Multicenter trial