*G*Power--Seeing Power Graphically*

## David C. Howell

There are several programs available for calculating power, and
among my favorites is DataSim (Bradley, 1988). At the moment DataSim is really only fully
functional for the Mac, but a Windows version will get here someday. But a very nice, and
free, program for both Macintosh and DOS computers is G*Power, by

Erdfelder, E., Faul, F. & Buchner, A. (in press). G*Power: A general power
analysis program. Behavior Research Methods, Instruments, & Computers.

Information on the availability of this program can be obtained at via WWW at:

http://www.psychologie.uni-trier.de:8000/projects/gpower.html

As a simple demonstration of what this program will do, the following graphic displays
power as a function of sample size for a oneway analysis of variance with four groups,
with an anticipated effect size of .40. (The heading in the Graph Window got messed up by
my own graphics program. It should read "Power(1 - beta) alpha
0.0500 etc.")

While I have plotted power as a function of sample size, I could have plotted several
other things, such as power as a function of effect size, effect size as a function of
sample size, and any other combinations of power, sample size, effect size, and alpha.
This program will work very nicely for lecture purposes, and will give students a good
grasp of the interrelationships among the parameters governing power.

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Last revised: 7/11/98