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Biostatistics 3:277-287 (2002)
© 2002 Oxford University Press

A sample size adjustment procedure for clinical trials based on conditional power

Gang Li, Weichung J. Shih, Tailiang Xie and Jiang Lu

Gang Li. Organon Inc., 375 Mt Pleasant Ave., West Orange, NJ 07052, USA G.Li{at}organoninc.com
Weichung J. Shih. University of Medicine and Dentistry of New Jersey, New Brunswick, NJ 08901, USA shihwj{at}umdnj.edu
Tailiang Xie. Whitehall-Robins Healthcare, Madison, NJ 07940, USA
Jiang Lu. University of the Witwatersrand, PO WITS, 2050, Johannesburg, South Africa

When designing clinical trials, researchers often encounter the uncertainty in the treatment effect or variability assumptions. Hence the sample size calculation at the planning stage of a clinical trial may also be questionable. Adjustment of the sample size during the mid-course of a clinical trial has become a popular strategy lately. In this paper we propose a procedure for calculating additional sample size needed based on conditional power, and adjusting the final-stage critical value to protect the overall type-I error rate. Compared to other previous procedures, the proposed procedure uses the definition of the conditional type-I error directly without appealing to an extra special function for it. It has better flexibility in setting up interim decision rules and the final-stage test is a likelihood ratio test.

Keywords: Conditional power; Interim analysis; Sample size re-estimation; Two-stage design


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