Subjects who withdraw from diet clinical trials are a drain on limited resources and reduce statistical power. Dropout pattern data, collected during a clinical trial for which the primary findings compared weight loss from three dieting protocols, are examined using survival analysis and found to be exponentially distributed. The predicted probability of remaining in the study is 83% for 30 days and 60% for 84 days. Survival analysis methods consider subjects who did not return after the initial visit and others who may have continued dieting beyond study termination. When applied to clinical trials, this type of analysis provides valuable information for planning and budgeting of future trials. Inclusion of a 1- to 2-week run-in period at the beginning of the study may improve retention. Otherwise, the diet researcher should consider increasing initial randomized sample size by approximately 10% to 25% as an allowance for early withdrawals.
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P. S. Landers is an assistant professor, Department of Nutritional Sciences, University of Oklahoma Health Sciences Center, Oklahoma City.
T. L. Landers is a professional engineer, associate dean, and Morris R. Pittman professor in the College of Engineering, University of Oklahoma, Norman.
© 2004 American Dietetic Association. Published by Elsevier Inc. All rights reserved.