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Sample Size Estimation: How Many Individuals Should Be Studied?¹

¹ From the Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University, 600 N Wolfe St, Central Radiology Viewing Area, Rm 117, Baltimore, MD 21287. Received December 17, 2001; revision requested January 29, 2002; revision received March 7; accepted March 13. Address correspondence to the author (e-mail: jeng@jhmi.edu).

The number of individuals to include in a research study, the sample size of the study, is an important consideration in the design of many clinical studies. This article reviews the basic factors that determine an appropriate sample size and provides methods for its calculation in some simple, yet common, cases. Sample size is closely tied to statistical power, which is the ability of a study to enable detection of a statistically significant difference when there truly is one. A trade-off exists between a feasible sample size and adequate statistical power. Strategies for reducing the necessary sample size while maintaining a reasonable power will also be discussed.

© RSNA, 2003

Index terms: Radiology and radiologists, research • Statistical analysis

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