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Randomized Control Trials

  1. William R. Shadish,
  2. Rodolfo Galindo

Published Online: 30 JAN 2010

DOI: 10.1002/9780470479216.corpsy0770

Corsini Encyclopedia of Psychology

Corsini Encyclopedia of Psychology

How to Cite

Shadish, W. R. and Galindo, R. 2010. Randomized Control Trials. Corsini Encyclopedia of Psychology. 1–2.

Author Information

  1. University of California, Merced

Publication History

  1. Published Online: 30 JAN 2010


Randomized control trials (RCTs) are experiments that randomly assign participants to two or more groups to assess the effects of treatment (Shadish, Cook, & Campbell, 2002). Random assignment, when properly implemented and analyzed, yields effect estimates that are both unbiased and consistent. They are unbiased in that the expectation of the effect equals the effect in the population; thus, while any single trial might not yield exactly the right answer, the average over many similar RCTs will approach the right answer. They are consistent in that the effect in any given randomized experiment will approach the population effect as the sample size of the experiment increases. Therefore, large sample sizes in any single trial make it more likely that the results will be accurate.


  • counterfactual;
  • control groups;
  • masking;
  • double blind;
  • attrition;
  • intent to treat