What random assignment does and does not do

J Clin Psychol. 2003 Jul;59(7):751-66. doi: 10.1002/jclp.10170.


Random assignment of patients to comparison groups stochastically tends, with increasing sample size or number of experiment replications, to minimize the confounding of treatment outcome differences by the effects of differences among these groups in unknown/unmeasured patient characteristics. To what degree such confounding is actually avoided we cannot know unless we have validly measured these patient variables, but completely avoiding it is quite unlikely. Even if this confounding were completely avoided, confounding by unmeasured Patient Variable x Treatment Variable interactions remains a possibility. And the causal power of the confounding variables is no less important for internal validity than the degree of confounding.

Publication types

  • Comparative Study
  • Review

MeSH terms

  • Causality*
  • Confounding Factors, Epidemiologic
  • Humans
  • Observer Variation
  • Psychotherapy
  • Random Allocation*
  • Randomized Controlled Trials as Topic / methods*
  • Randomized Controlled Trials as Topic / statistics & numerical data
  • Reproducibility of Results
  • Selection Bias
  • Stochastic Processes*
  • Treatment Outcome*