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Sample size: Confidence Interval for a difference between means


Calculates the required minimum sample size for the estimation of a confidence interval with a required width for a difference between two independent means (Machin et al., 2009).

Note that the calculation does not include a null hypothesis value or a factor for power (1−β). Therefore the estimated sample size does not give a certainty that a particular value will fall inside or outside the confidence interval. The number of cases is only the number required to attain a specified confidence interval width.

Required input

  • Confidence level (%): select the confidence level: 90, 95 or 99%. A 95% confidence level (the value for a 95% Confidence Interval) is the most common selection. You can enter a different confidence level if required.
  • Pooled standard deviation: you can find the pooled standard deviation in the output of the Independent samples t-test (see also Independent samples t-test: computational notes).
  • Confidence interval width (2-sided): this is the required total width of the confidence interval. For example when a difference is 25 with 95% Confidence Interval 20 to 30, then the confidence interval width is 10.
  • Ratio of sample sizes in Group 1 / Group 2: enter 1 for equal sample sizes in both groups. Enter 2 if the number of cases in group 1 must be double of the number of cases in group 2.


Sample size for the confidence interval of a difference between two means


  • Machin D, Campbell MJ, Tan SB, Tan SH (2009) Sample size tables for clinical studies. 3rd ed. Chichester: Wiley-Blackwell.

See also