JSM 2004 - Toronto

Abstract #301666

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Activity Number: 164
Type: Contributed
Date/Time: Monday, August 9, 2004 : 2:00 PM to 3:50 PM
Sponsor: Biometrics Section
Abstract - #301666
Title: Symmetrized Percent Change for Treatment Comparisons
Author(s): Gregory D. Ayers*+ and Donald Berry
Companies: University of Texas M.D. Anderson Cancer Center and University of Texas M.D. Anderson Cancer Center
Address: Dept. of Biostatistics and Applied Mathematics, Houston, TX, 77030-4009,
Keywords: change from baseline ; expanded baseline eligibility ; measure of treatment effect ; percent change ; required sample size ; robust percent change
Abstract:

A standard way of addressing treatment effect is to relate post-treatment (F) with the corresponding baseline (B) measurement. Commonly used measures of effect are absolute difference, D = F-B, and percent change, PC = 100*(F-B)/B. A less commonly used measure was suggested by Berry (1989): symmetrized percent change, SPC=100*(F-B)/(B+F). We examine the interpretability and relative performances of these three measures in the context of a parallel, two-armed clinical trial. The conclusions are sometimes the same for all three measures. But they may well be different. We show by simulating a realistic setting that analyses based on SPC tend to be more powerful than those employing the other two measures. For the cases we consider the savings in sample size range from 20-70% when using SPC as compared with using one of the other measures.

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