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.
Bouguen, Adrien et al. (2019) Using randomized controlled trials to estimate long-run impacts in development economics, Annual Review of Economics, vol. 11, pp. 523–561.