JSM 2004 - Toronto

Abstract #301864

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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 - #301864
Title: On Appropriate Sample Size for Using Normal Approximation in Clinical Trials
Author(s): Jun Zhao*+ and Gang Li
Companies: Organon Pharmaceuticals USA Inc. and Organon Pharmaceuticals USA Inc.
Address: 56 Livingston Ave., Roseland, NJ, 07068,
Keywords: sample size ; skewness ; Type I error ; normal approximation ; p value
Abstract:

To evaluate the effectiveness of study drug in clinical trials, p values for hypothesis testing are often obtained by using normal approximation, such as in t-test, ANOVA, and chi-square test. But clinical data are rarely normally distributed. A question often raised is how large a sample is appropriate for using normal approximation to obtain p values. One important factor that hinders normal approximation is the skewness of data. The larger the skewness, the greater the sample size is needed for normal approximation. Instead of examining the approximation on a whole distribution function, our focus is on the tail probability (the Type I error), which is used as a criterion to evaluate the difference of treatment effects. For each level of skewness, a minimal sample size for which the normal approximation approach can control the Type I error is suggested.

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