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Related Experiment Videos

Random versus nonrandom assignment in controlled experiments: do you get the same answer?

W R Shadish1, K Ragsdale

  • 1Department of Psychology, University of Memphis, Tennessee 38152, USA.

Journal of Consulting and Clinical Psychology
|December 1, 1996
PubMed
Summary
This summary is machine-generated.

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Psychotherapy meta-analyses combining random and nonrandom assignment studies may skew results. Randomized trials show higher effects than nonrandomized ones, though differences narrow with covariate adjustments.

Area of Science:

  • Psychology
  • Clinical Psychology
  • Research Methodology

Background:

  • Psychotherapy meta-analyses often pool data from randomized and nonrandomized controlled trials.
  • The impact of differing assignment methods on meta-analytic outcomes is not well understood.

Purpose of the Study:

  • To investigate whether random versus nonrandom assignment in psychotherapy outcome studies influences meta-analytic findings.
  • To compare the effect sizes and variability between randomized and nonrandomized marital and family therapy studies.

Main Methods:

  • A comparative analysis of 64 randomized and 36 nonrandomized studies in marital and family therapy outcome research.
  • Statistical examination of mean post-test effects and effect size variability.
  • Covariate analysis including pretest effect sizes and control group characteristics.

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Main Results:

  • Randomized assignment studies consistently demonstrated higher mean post-test effects and lower variability compared to nonrandomized studies.
  • Covariate adjustments, particularly pretest effect size and control group features, reduced the observed difference by approximately 50%.

Conclusions:

  • Nonrandomized studies may approximate randomized experiment results under specific conditions.
  • Randomized controlled trials remain the gold standard for psychotherapy research due to their reliability and precision.