The Randomized Controlled Trial

A randomized controlled trial is a controlled trial in which subjects are assigned to the treatment and comparison groups by chance rather than by the experimenter's or the participant's own choice, so that known and unknown differences between people are spread evenly across both groups and cannot themselves explain a difference in outcome. This atlas carries it as a concept distinct from the general controlled trial (proposed by James Lind in 1747, minted separately): the controlled trial only requires a comparison group; randomization is a specific, later, and separately credited methodological innovation for assigning who goes in which group. The Medical Research Council's 1948 trial of streptomycin for pulmonary tuberculosis, designed by the statistician Austin Bradford Hill, is widely credited as the first trial in which patients were allocated to treatment groups using a formal random-numbers procedure, and it is now considered the methodological ancestor of the modern randomized controlled trial, medicine's standard for testing whether a treatment works.