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G*Power for Macintosh, Version 2.1.2 September 1997

Contents of this document:

(1) Installation and registration

(2) What G*Power does and how it works

(3) How to get G*Power (i.e., where to check for future updates and versions for
other platforms)

(4) How to find help for using G*Power

(5) Address for correspondence

(6) Disclaimer

A more detailed description of the program and its background is available
in printed form:

Erdfelder, E., Faul, F., & Buchner, A. (1996). GPower: A general power
analysis program. Behavior Research Methods, Instruments, & Computers,
28, 1-11.

This article should appear very soon.

(1) Installation and registration

There are two versions of G*Power:

G*Power PPC

is the version to use on Power Macintoshes.

G*Power 68K

should run on any 68K Macintosh using System 7 or higher. It has also been tested
successfully on some Power Macintosh models on which it runs in emulation mode.

Sorry, earlier system software versions are no longer supporeted, and a special
verion of G*Power for 68K computers with a 68881 FPU is also no longer available.

No special installation procedure is necessary to use G*Power. Simply copy the
program to your harddisk. However, we recommend that you install only the
version appropriate for your Macintosh. Remove the other versions.

G*Power is free. You may to give the program to friends and colleagues who might
find it useful.

VERY IMPORTANT: If you want to include G*Power on a Shareware or Freeware CD-ROM
you MUST ask the authors for permission. Contact us under the address given

Note that there is an MS-DOS version of G*Power, so if your friends and
colleagues use that platform, they can also work with G*Power if they like (see
Section (3) of this file for the FTP site to check for updates and the MS-DOS
version of G*Power).

We encourage you to let us know your complete address (including your e-mail
address if possible) and the version of G*Power you use. As a registered user,
you will be informed about future program revisions. Your personal data will be
used for this purpose only. Please send this information to:

Axel Buchner
FB I - Psychologie
Universität Trier
D-54286 Trier


We also ask you to report any errors or problems encountered with G*Power to the
address mentioned in Section (5) of this file.

What follows is a brief description of the program (it takes no more than five
minutes to read it).

(2) What it does and how it works

G*Power performs high-precision statistical power analyses for the most common
statistical tests in behavioral research, that is, t-, F-, and Chi2-tests. The
power values obtained by "accuracy mode" calculations of G*Power are correct up
to five significant digits at least. Although available for taking a quick look
at a problem, we do not encourage "speed mode" calculations because they are less

G*Power computes

(a) power values for given sample sizes, effect sizes and alpha-levels (post hoc
power analyses);

(b) sample sizes for given effect sizes, alpha-levels, and power values (a priori
power analyses);

(c) alpha- and beta-values for given sample sizes, effect sizes and
beta/alpha-ratios (compromise power analyses).

It also displays graphically the relation between any two of the relevant

Note that in a priori power analyses, the resulting sample size is
rounded to the next larger integer. Therefore, the actual power of
these analyses is usually somewhat larger than the one specified.
G*Power informs you how large the power of your test actually is given
the calculated samplesize.

In addition, G*Power calculates for you the critical value of the
relevant test statistic and the noncentrality parameter of the
noncentral distribution of your test statistic.

----------------------- The "Analysis window" -----------------------

The "Analysis window" is used to specify the information necessary for a
particular power analysis, and it displays the results of the last analysis.

Click "Calculate" to perform a single analysis for the Type of Power Analysis,
the Type of Test, and the parameters you specified.

Click "Calc ES" (where ES is d, r, f, f2, or w, depending on the type of test
currently selected) to open a dialog in which you can specify the values
necessary to calculate the effect size for your power analysis from empirical
data (e.g., from published studies).

Click "Draw graph" to open a dialog in which you can specify the values necessary
to display one parameter (e.g., the power) as a function of another parameter
(e.g., the total sample size) over a range of values (e.g., from 20 to 220
subjects) in the "Graph window".

If the "Graph window" is already open, you can simply double-click this window to
open the dialog in which you can specify the graph parameters.

------------------------- The "Graph window" -------------------------

The "Graph window" can be used to display one parameter (e.g., the power) as a
function of another parameter (e.g., the total sample size) over a range of
values (e.g., from 20 to 220 subjects).

The header of the graph always specifies the type of test, and the fixed values.
The note below the graph tells you what calculation mode you used. Default is
"Accuracy mode" which is very accurate (up to five significant digits at least)
but the price is that you will usually have to wait a little longer until you see
a result. "Speed mode" calculations are fast but they may not be very accurate
(particularly for 'extreme' values). If you selected "Speed mode", do not trust
the results too much!

You can copy the graph using "Copy graph" from the "Edit" menu and paste it into
your word processor or drawing program documents for saving or editing the graph
information. Inside G*Power you cannot edit or save a graph.

The scaling of the axes in the "Graph window" depends on the size of the window,
so re-sizing the window may have the effect of a more fine-grained scaling.

If the "Protocol (table)" option in the "Graph dialog" is on, a table of the
values used to plot the graph is written to the Result window. This table can be
imported into your favorite graphics package, either via the clipboard or by
saving the table as a G*Power file (see below).

------------------------- The "Result window" -------------------------

If the "Result window" is open, all parameters of all power analyses performed in
the "Analysis window" are recorded here.

The "Result window" is fully editable (using the commands from the Edit menu).
You can either copy the values you need to the clipboard, or you can save the
entire information of the "Result window" to a G*Power file using the Save or
Save as commands from the File menu.

Double-clicking such a file launches G*Power and opens the file so you can record
more analyses in it. However, because G*Power files are of type TEXT, they may
also be opened from inside your favorite word processor or graphics package.

(3) How to get G*Power

The most convenient way to get a copy of G*Power is to download the program from
the public FTP server at the University of Trier, Germany. This is where you
should check for updates and future releases, and for versions for other
platforms of G*Power. However, we will also try to make available updates and
future releases of the Macintosh version on an info-mac archive near you.

There are two ways to access this server:

(I) Via WWW:

(II) Via FTP:

(a) if you want the Macintosh-version of G*Power: (the program)

(b) if you want the MS-DOS-version of G*Power: (German version) (international version)

Set the file type to binary (omitting to set the file type properly is a frequent

Convert the compressed file to a standard Macintosh or DOS file:

(a) Macintosh-version: Use StuffIt or the Stuffit Expander. No additional
installation is necessary.

(b) DOS-version (self-extracting archive): GPower2i.exe (international version)
or GPower2d.exe (German version). This will create a subdirectory GPower in your
current working directory which will contain all necessary files.

(4) How to find help for using G*Power

We have made available for you a www-based guide to G*Power at

(5) Address for correspondence

Please report bugs and suggestions to improve the program to:

Axel Buchner
FB I - Psychologie
Universität Trier
D-54286 Trier


(6) Disclaimer

We hereby disclaim all warranties relating to this software, whether expressed or
implied, including without limitation any implied warranties of merchantability
or fitness for a particular purpose. We will not be liable for any special,
incidental, consequential, indirect or similar damages due to loss of data or any
other reason, even if we or agents of ours have been advised of the possibility
of such damages. In no event shall we be liable for any damages, regardless of
the form of the claim. The person using the software bears all risk as to the
quality and performance of the software.

Original file name: G*Power Read Me - converted on Thursday, 23 December 1999, 23:30

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