Estimating sample sizes for binary, ordered categorical, and continuous outcomes in two group comparisons
Open accessAbstract
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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The available sample size was judged optimal for identifying factors associated with iron supplementation utilization using ordinal logistic regression with 95% confidence level, 90% power and small effect size (OR of 1.5).
“Further, based on the power calculation formula [14] designed for ordinal logistic regression analysis, the available sample size is judged to be optimal for identifying the factors associated with utilization of iron supplementation with 95% confidence level and 90% power and small effect size (Odds Ratio (OR) of 1.5) to be detected.”
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- Authors
- Campbell, M. J., Julious, S. A., & Altman, D. G.
- Affiliations
- Southampton General Hospital (GB)
- Corresponding author
- Campbell, M J
- Venue
- BMJ
- Publisher
- BMJ
- Published
- 1995-10-28
- Volume
- 311
- Issue
- 7013
- Pages
- 1145-1148
- ISSN
- 0007-1447 (ISSN-L)
- 0959-8138
- 1468-5833
- PMC
- PMC2551061
- OpenAlex
- W2076470946
- Cited by
- 476
- References
- 3
- Citations by year
- 2026: 8 · 2025: 12 · 2024: 11 · 2023: 18 · 2022: 15 · 2021: 13 · 2020: 14 · 2019: 17 · 2018: 19 · 2017: 16 · 2016: 21 · 2015: 18 · 2014: 18 · 2013: 27 · 2012: 7
- FWCI
- 9.41
- Type
- journal article
- Language
- en
- Open access
- green
- www.ncbi.nlm.nih.gov
- Licence
- www.bmj.com/company/legal-information/terms-conditions/legal-information/tdm-licencepolicy (tdm)
- Locations
- BMJ (publishedVersion)
- PubMed Central (publishedVersion)
- PubMed (publishedVersion)
- HighWire Press Open Archive (submittedVersion)
- Topics
- Physical Sciences › Mathematics › Statistics and Probability › Statistical Methods in Clinical Trials
- Physical Sciences › Mathematics › Statistics and Probability › Statistical Methods in Epidemiology
- Physical Sciences › Mathematics › Statistics and Probability › Probability and Statistical Research
- Keywords
- sample size determination, two-group comparison, binary outcomes, continuous outcomes, statistical power, unequal group sizes, research design
- MeSH terms
- Data Interpretation, Statistical, Humans, Sample Size
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