"Piface" the cow, by Brian Lenth Java applets for power and sample size
Select the analysis to be used in your study:

Note: For security reasons, it is best to access this page using the HTTPS protocol: https://homepage.divms.uiowa.edu/~rlenth/Power/index.html. Also add https://homepage.divms.uiowa.edu to the list of authorized sites in your Java configuration.

This software is intended to be useful in planning statistical studies.  It is not intended to be used for analysis of data that have already been collected.

Each selection provides a graphical interface for studying the power of one or more tests.  They include sliders (convertible to number-entry fields) for varying parameters, and a simple provision for graphing one variable against another.

Each dialog window also offers a Help menu (on Macs, the Options and Help menus are added at the top of the screen). Please read the Help menus before contacting me with questions.

The "Balanced ANOVA" selection provides another dialog with a list of several popular experimental designs, plus a provision for specifying your own model.

Note: The dialogs open in separate windows. If you're running this on an Apple Macintosh, the applets' menus are added to the screen menubar -- so, for example, you'll have two "Help" menus there!

You may also download this software to run it on your own PC.

Note: These require a web browser capable of running Java applets (version 1.3 or higher). If you do not see a selection list above, chances are that you either have disabled Java, or you have an outdated  implementation of Java. In the latter case, you need to download and install the JRE plug-in from www.oracle.com/technetwork/java/
Due to a compatibility bug, many plug-ins size the applet window before allowing for an additional strip with a security warning.; to compensate, drag the bottom of the window downward a bit.

Please read this comment

I receive quite a few questions that start with something like this:
"I'm not much of a stats person, but I tried [details...] -- am I doing it right?"
Please compare this with:
"I don't know much about heart surgery, but my wife is suffering from ... and I plan to operate ... can you advise me?"
Folks, just because you can plug numbers into a program doesn't change the fact that if you don't know what you're doing, you're almost guaranteed to get meaningless results -- if not dangerously misleading ones. Statistics really is like rocket science; it isn't easy, even to us who have studied it for a long time. Anybody who think it's easy surely lacks a deep enough knowledge to understand why it isn't! If your scientific integrity matters, and statistics is a mystery to you, then you need expert help. Find a statistician in your company or at a nearby university, and talk to her face-to-face if possible. It may well cost money. It's worth it.

If you're blocked by a security setting

You may get into a situation, especially after updating Java, where this applet is blocked by a security setting in your system. In that case, in order to run the applets, you need to this site to the list of exceptions to your security rules. To do this in Windows, find "Configure Java" on the Start menu, go to the security tab, and near the bottom, click on the "Edit site list" button, and add "https://homepage.divms.uiowa.edu" to the list. Then things should work.


If you have questions about sample size or power, I suggest going to Cross Validated which is frequented by a large number of people. The answer you need may already be available -- so do a search first!

Citing this software

If you use this software in preparing a research paper, grant proposal, or other publication, I would appreciate your acknowledging it by citing it in the references.  Here is a suggested bibliography entry in APA or "author (date)" style:

Lenth, R. V. (2006-9).  Java Applets for Power and Sample Size [Computer software].  Retrieved month day, year, from http://www.stat.uiowa.edu/~rlenth/Power.

This form of the citation is appropriate whether you run it online (give the date you ran it) or the stand-alone version (give the date you downloaded it).

Download to run locally

The file piface.jar may be downloaded so that you can run these applications locally.  [Note: Some mail software (that thinks it is smarter than you) renames this file piface.zip.  If this happens, simply rename it piface.jar; do not unzip the file.]  You may also want the icon file piface.ico if you put it on your desktop or a toolbar.  You will need to have the Java Runtime Environment (JRE) or the Java Development Kit (JDK) installed on your system.  You probably already have it; but if not, these are available for free download for several platforms from Sun.  If you have JDK or JRE version 1.2 or later, then you can probably run the application just by double-clicking on piface.jar.  Otherwise, you may run it from the command line in a terminal or DOS window, using a command like

    java -jar piface.jar

This will bring up a selector list similar to the one in this web page.  A particular dialog can also be run directly from the command line, if you know its name (can be discovered by browsing piface.jar with a zip file utility such as WinZip).  For example, the two-sample t-test dialog may be run using

java -cp piface.jar rvl.piface.apps.TwoTGUI

Frequently Asked Questions


Here are two very wrong things that people try to do with my software:
Here are three very right things you can do:
Many funding agencies require a power/sample-size section in grant proposals.  Following the above guidelines is good for improving your chances of being funded.  You will have established that you have thought through the scientific issues, that your procedures are sound, and that you have a defensible sample size based on realistic variance estimates and scientifically tenable effect-size goals. 

To read more, please see the following references:


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The views and opinions expressed in this page are strictly those of the page author.
The contents of this page have not been approved by Mathematical Sciences, the College of Liberal Arts, or The University of Iowa.


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