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

BMJ. 1995 Oct 28;311(7013):1145-8. doi: 10.1136/bmj.311.7013.1145.


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.

Publication types

  • Research Support, Non-U.S. Gov't
  • Review

MeSH terms

  • Data Interpretation, Statistical
  • Humans
  • Sample Size*