MedCalc

# Sample size: Confidence Interval for a mean difference between paired samples

 Command: Sample sizeConfidence Interval estimation & PrecisionMean difference between paired samples

## Description

Calculates the required minimum sample size for the estimation of a confidence interval with a required width for an average difference between paired observations (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.
• Standard deviation of differences: the expected standard deviation of the differences between the paired observations (known for example from a Paired samples t-test in previous studies, or from the literature).
• Confidence interval width (2-sided): this is the required total width of the confidence interval. For example when a mean difference is 35 with 95% Confidence Interval 30 to 40, then the confidence interval width is 10.

## Literature

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