John Eng, M.D.
The Russell H. Morgan Department of Radiology and Radiological Science
Johns Hopkins University, Baltimore, Maryland, USA
Computer requirements for this page: Reasonably up-to-date web browser
and Java versions are required. Due to its continuing
evolution, the latest Java version
is highly recommended. Note that Java is no longer installed by default on some new
computers, so manual installation of Java may be required.
Please send any bugs, questions, comments, or suggestions to
All electronic mail will be answered.
Instructions: This page calculates the sample size for four simple study designs. The equations
are discussed more fully in Ref. 1, and this Web page is intended to accompany that article. To estimate
a sample size, identify the equation appropropriate to your study design. Enter values for the parameters
under "Input Values" and click the corresponding "Calculate" button. See
below for program development details, acknowledgments,
and a list of references.
Program Development Details and Acknowledgments: The lavender box at the top of this
page contains StatSupport, a program that supplies basic numerical functions for performing
statistical calculations on the Web. StatSupport is written in the Java programming language to enable its use over the
Web. Ref. 2 documents the algorithm for StatSupport's inverse normal distribution routine, which was used
to calculate Eqs. 1 and 2.
Eng J. Sample size estimation: how many individuals should be studied? Radiology 2003; 227: 309-313.