Sample Size Calculation for Before-After Experiments with Partially Overlapping Cohorts

• Activity Number: 458
• Type: Contributed
• Date/Time: Wednesday, August 6, 2014 : 8:30 AM to 10:20 AM
• Sponsor: Biopharmaceutical Section
• Author(s): Song Zhang*+ and Jing Cao and Chul Ahn
• Companies: University of Texas Southwestern Medical Center and Southern Methodist University and University of Texas Southwestern Medical Center
• Keywords: GEE ; Sample Size ; before-after

Abstract

We investigate sample size calculation for before-after experiments where the out- come of interest is binary and the enrolled subjects contribute a mixed type of data: some subjects contribute complete pairs of before- and after-intervention outcomes, while some subjects contribute incomplete data (before-intervention only or after- intervention only). We treat the incomplete observations as missing data in a general- ized linear model and the GEE approach is employed to derive a closed form sample size formula. The impacts of various designing factors are appropriately accounted for in the sample size formula, including intervention effect, baseline response rate, within-subject correlation, and distribution of missing values in the before- and after- intervention periods. We conduct simulation studies to demonstrate that the proposed sample size maintain the nominal power and type I error under a wide spectrum of trial configurations. A real application example is presented.