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October 28, 1995BMJ480 citationsOpen Access

Estimating sample sizes for binary, ordered categorical, and continuous outcomes in two group comparisons

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MCMichael J. CampbellSJSteven A. JuliousDADavid Altman

Key Points

  • The aim is to provide clear methodologies for calculating sample sizes in two group studies for different types of outcomes.
  • Outlined formulas for sample size calculations for binary, ordered categorical, and continuous outcomes.
  • Provided worked examples to demonstrate practical applications.
  • Discussed the impact of unequal group sizes on power.
  • Maximum statistical power is typically achieved with equal group sizes.
  • Calculations for scenarios with unequal group sizes are included for comprehensive guidance.

Abstract

Sample size calculations are now mandatory for many research protocols, but the ones useful in common situations are not all easily accessible. This paper outlines the ways of calculating sample sizes in two group studies for binary, ordered categorical, and continuous outcomes. Formulas and worked examples are given. Maximum power is usually achieved by having equal numbers in the two groups. However, this is not always possible and calculations for unequal group sizes are given.

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Cite This Study

Campbell et al. (1995) studied this question.

synapsesocial.com/papers/6a08589cdf3db87398108c1ahttps://doi.org/10.1136/bmj.311.7013.1145
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Also Consider

Synapse has enriched 5 closely related papers on similar clinical questions. Consider them for comparative context:

  1. 1Going beyond simple sample size calculations: a practitioner's guide2025 · 3 citations
  2. 2Optimal sample size allocation for two‐arm superiority and non‐inferiority trials with binary endpoints2024 · 1 citations
  3. 3An Empirical Assessment of the Cost of Dichotomization of the Outcome of Clinical Trials2026 · 5 citations
  4. 4Sample size calculations in health research: Contemporary issues and practices2022
  5. 5Exact power and sample size in clinical trials with two co-primary binary endpoints2025