Calculating standard deviation of difference for determination of sample size for planned paired t-test analysis.

Introduction: For pre-post and cross-over design analysis of numerical data, paired t-test is the simplest analysis to perform. In planning such studies, it is imperative to calculate appropriate sample size required for the test to detect hypothesized difference. However, the sample size formula requires determination of standard deviation of difference, which is not commonly reported. In this article, the author guides the reader to calculation of standard deviation of difference from standard deviation of each separate occasion.