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Related Experiment Videos

Small sample failure of random assignment: a further examination.

M J Strube1

  • 1Department of Psychology, Washington University, St. Louis, Missouri 63130.

Journal of Consulting and Clinical Psychology
|April 1, 1991
PubMed
Summary
This summary is machine-generated.

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Random assignment creates equivalent groups, but its effectiveness relies on sample size. Small samples may show nonequivalence, but erroneous inferences are unlikely, though other issues persist.

Area of Science:

  • Psychology
  • Research Methodology

Background:

  • Random assignment is crucial for establishing equivalence between experimental groups.
  • Its effectiveness is influenced by sample size, with larger samples yielding greater equivalence.

Purpose of the Study:

  • To evaluate the impact of sample size on the effectiveness of random assignment.
  • To assess the likelihood of erroneous inferences due to nonequivalence in small samples.

Main Methods:

  • The study theoretically demonstrates the relationship between sample size and the probability of nonequivalence on nuisance variables.
  • It analyzes the probability of erroneous inferences, specifically Type I errors, in the context of small sample sizes.

Main Results:

  • While small samples increase the likelihood of nonequivalence on nuisance variables, the probability of this leading to erroneous inferences is low.

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  • The probability of erroneous inferences in the absence of a true effect is comparable to the nominal Type I error rate.

    • Conclusions:

    • Small sample sizes in psychotherapy research are unlikely to have biased inferences regarding treatment effectiveness.
    • Despite low risk of biased inferences, small samples present other methodological challenges that discourage their routine use.