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Most psychological studies have adequate sample sizes for large effect sizes. However, achieving desired power and alpha levels for medium or small effect sizes may require more participants than typically used in research.

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Area of Science:

  • Psychology
  • Statistical Power Analysis

Background:

  • Determining appropriate sample sizes is crucial for the validity of psychological research.
  • Statistical power analysis helps ensure studies can detect true effects.

Purpose of the Study:

  • To provide tables of alpha values based on sample size, effect size, and desired power.
  • To assess the adequacy of sample sizes in psychological studies for various effect sizes.

Main Methods:

  • Presented tables detailing alpha values across different sample sizes, effect sizes (small, medium, large), and power levels.
  • Calculated required participants for specific alpha (0.05) and power (90%) levels.

Main Results:

  • Sample sizes in many psychological studies are sufficient for detecting large effect sizes (0.8).
  • Achieving 90% power with a 0.05 alpha level requires approximately 70 participants in two groups for large effects.
  • Detecting medium effect sizes likely requires around 170 participants, potentially exceeding typical study sizes.
  • Small effect sizes are infrequently studied with adequate participant numbers or statistical power.

Conclusions:

  • While large effect sizes are often adequately powered, medium and small effect sizes may be underpowered in psychological research.
  • Researchers should carefully consider sample size requirements for their specific effect size and desired statistical power.
  • The findings highlight potential limitations in the power of studies examining smaller effects in psychology.