A SAS Module for Sample-Size Analysis
Designed and Written by Ralph O'Brien
Department of Biostatistics and Epidemiology
Cleveland Clinic Foundation
20 September 2003
UnifyPow version 2002.08.17a
You can download a new version of UnifyPow:
UnifyPow version 2002.08.17a (UnifyPow020817a.sas). This plain text file is
distributed so that it runs by %INCLUDE-ing it as I have been
demonstrating in the workshops.
Here also are the notes (PDF) that I covered in my workshop at the
Joint Statistical Meetings in New York City in August 2002.
UnifyPow Notes version 2002.08.11
(UnifyPowNotes020811.pdf). This is designed to be printed
Chris Skibinski has developed GraphPow, which (as you might guess)
graphs UnifyPow results using SAS/GRAPH. Click here
to visit her website for the code and more information.
New Tools from SAS Institute
SAS Institute developer Dr. John
Castelloe and his colleagues are designing and building products in
this area that should eventually make UnifyPow obsolete. I am
consulting actively and formally on all this, and I hope to now turn
my attention to writing about sample-size analysis (with John) instead
of programming and supporting UnifyPow.
The first products are:
* PROC POWER handles many of the basics.
* PROC GLMPOWER will handle linear models using syntax and modeling structure
congruent with PROC GLM.
* Java-based interface that works from your web browser.
Experimental versions of these have been released in version 9.0, which
has gone to selected sites. Version 9.1 will be the first full release
of SAS 9.x, and it will contain production versions of these tools, which will have even greater functionality.
documentation is available
in PDF from SAS.
Note: The rest of this site needs to be updated, but there is never enough
time for this anymore. Sorry.
UnifyPow is a freeware SAS module/macro that performs power
analysis and other matters related to statistical planning for many
types of common research designs and data analytic methods. It is
described in O'Brien (1998), and is being demonstrated through various
workshops and convention presentations. Don't misjudge UnifyPow
because it is free: Its ease-of-use, capabilities, flexibility, a
numerical accuracy are considerable. See commentary by Nicholas Petreley.
It is important that you notify me if you have
concerns about anything. See quote by Fred Brooks.
Before downloading the files, it is important that you add your name
and email address to the UnifyPow mailing list. Doing so will allow me
to keep you informed about future releases of the freeware and its
documentation. Registering is optional, of course, and I will not
release the list or use it for any other purpose. Please
click here to register.
Files for downloading
The following files may be downloaded by clicking on the
filenames. *.pdf files need to be viewed and printed with Acrobat
Reader, a breakthrough free tool that is commonly used on Windows
3.1/95/NT, Macintosh, and many varieties of UNIX. I have found that
downloading Acrobat Reader 4.xx and installing it were easy tasks.
Note: versions of Acrobat Reader earlier than 3.xx have failed to read
Tour of UnifyPow: most recent update of O'Brien (1998),
the proceedings paper for the invited presentation at SUGI 23, March
1998, Nashville, TN. This describes the current and planned
capabilities of UnifyPow and demonstrates its use. As UnifyPow
evolves, this paper is updated and made available from this site.
ReadMe file on UnifyPow: tells how to get started. This does not
summarize UnifyPow's functionality and syntax. A plain text file.
UnifyPow.sas source code (module version): current release of UnifyPow all in one %include module. This plain text file is
distributed so that it runs as I have been demonstrating in the
workshops. Technically, it is not a true macro in this form, but there
are instructions in the opening lines to easily convert it to one.
(I find no compelling reason to do this.)
Input (.sas) file with numerous examples (module version):
all examples used in the current one-day workshop and other talks and
presentations. A plain text file.
Output file with numerous examples: output
(listing) produced by above input file. A plain text file.
Workshop notes: notes (slides) from the current one-day
workshop. This is not as good as a real manual, but hundreds of people
have learned UnifyPow (relatively painlessly) by studying this. An
Acrobat (.pdf) file.
Cases: a few case studies (more fully described examples)
distributed in the current workshop. An Acrobat (.pdf) file.
Final draft version of O'Brien and Muller (1993) chapter:
final manuscript that led to O'Brien RG, Muller KE (1993), "Unified
power analysis for t-tests through multivariate hypotheses," Chapter 8
(pp 297-344) in "Applied Analysis of Variance in Behavioral Science,"
edited by Lynne K. Edwards and published by Marcel Dekker. I've
updated the appendix to make it current with respect to freeware. An
Acrobat (.pdf) file.
Notes from my JSM-98 invited talk on generalized linear
models. This covers power analyses for problems that fall
within the generalized linear model, thus covering maximum likelihood
logistic regression, Poisson regression, etc. The strategy is a direct
extension of the method put forth in my 1986 SUGI paper on log-linear
models (summarized, with my permission, by Alan Agresti in his
terrific book, Categorical Data Analysis). I invited G. Shieh to join
me in this 1998 effort and, to my dismay, he unilaterally published
the essence of our work in a nice paper in Biometrics (December
2000). An Adobe Acrobat (.pdf) file.
Manuscript by O'Brien & Shieh on a "Pragmatic, Unifying Algorithm
(That) Gives Power Probabilities for Common F Tests of the
Multivariate General Linear Hypothesis." I have
stopped feeling guilty about not publishing this, because the few
people who work in this area all seem to know about it and accept its
content. An Adobe Acrobat (.pdf) file.
Downloading files using your Internet browser Use either
Netscape Navigator or Microsoft Internet Explorer. Here is what I have
experienced myself; yours may be a bit different.
Plain text files:
- Clicking on a plain text file (e.g., ReadMe.power) puts the contents
directly onto the screen.
- Doing a "Save As..." loads the file onto the local computer's hard
disk. Specify either "text" or "source." If you are running SAS on a
PC/Windows machine, you may want to store UnifyPow.sas in your SAS
- These plain text files can been read successfully with most
editors and word processors in UNIX, Mac OS, and PC/Windows platforms.
(Note: For the Mac, I encourage you to use the freeware BBEdit Lite. One of
the usual Mac applications for this, SimpleText, will not read these
- The Acrobat (*.pdf) files (e.g., Workshop.pdf) do not come up
automagically in the browser's window unless the local computer is set
up to link Acrobat Reader/Exchange with your browser. If not, the
browser either asks whether/where you want to save the file directly
onto your hard disk or it just saves it to someplace "obvious."
- View and print with Acrobat Reader/Exchange.
- If the image does automagically come up in the browser' window
(because it is linked to Acrobat Reader/Exchange), you probably will
still need to "Save As ..." the *.pdf file to your local hard
disk. *.pdf files are binary; when saving them, specifying either
"text" or "source" worked for me.
Thank you for your interest in my work.
Ralph G. O'Brien, PhD
O'Brien RG (1998), A Tour of UnifyPow: a SAS Module/Macro for
Sample-Size Analysis," Proceedings of the 23rd Annual SAS Users Group
International Conference, Cary NC: SAS Institute Inc., 1346-1355. You
download final update of this paper (Acrobat file).