Randomized controlled trials (RCTs) randomly separate experimental subjects into multiple groups, treat those groups differently, and compare the results. Most commonly, one group receives the treatment being tested, while the other receives a placebo or no treatment at all.
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Karnofsky, Holden (2012) How we evaluate a study, The GiveWell Blog, August 23 (updated 2 September 2016).
Ogden, Timothy (2020) RCTs in development economics, their critics and their evolution, in Florent Bédécarrats, Isabelle Guérin & François Roubaud (eds.) Randomized Control Trials in the Field of Development, Oxford: Oxford University Press, pp. 126–151.