======================================================================== G€Power for Macintosh, Version 2.1.1 April 1996 ======================================================================== 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) Disclaimer A more detailed description of the program and its background will be available soon 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 three versions of G€Power: G€Power PPC is the version to use on Power Macintoshes. It is amazingly fast. G€Power 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. G€Power FPU is useful if your 68K Macintosh is equipped with a 68881 or 68882 arithmetic coprocessor (G€Power/FPU is several times faster than G€Power but it will not run on machines without FPU, both 68K or PowerMacs). Again, you must have System 7 or higher. 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 below. -------------------------------------------------------------------------------- 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 Germany E-mail: buchner@cogpsy.uni-trier.de We also ask you to report any errors or problems encountered with G€Power to the address mentioned in Section (4) 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 ³accuracy mode² 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 ³speed mode² calculations because they are less accurate. 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 variables. 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 ³Analysis window² ----------------------- The ³Analysis window² is used to specify the information necessary for a particular power analysis, and it displays the results of the last analysis. Click ³Calculate² to perform a single analysis for the Type of Power Analysis, the Type of Test, and the parameters you specified. Click ³Calc ES² (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 ³Draw graph² 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 ³Graph window². If the ³Graph window² is already open, you can simply double-click this window to open the dialog in which you can specify the graph parameters. ------------------------- The ³Graph window² ------------------------- The ³Graph window² 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 ³Accuracy mode² 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. ³Speed mode² calculations are fast but they may not be very accurate (particularly for Œextreme¹ values). If you selected ³Speed mode², do not trust the results too much! You can copy the graph using ³Copy graph² from the ³Edit² 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 ³Graph window² depends on the size of the window, so re-sizing the window may have the effect of a more fine-grained scaling. If the ³Protocol (table)² option in the ³Graph dialog² 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 ³Result window² ------------------------- If the ³Result window² is open, all parameters of all power analyses performed in the ³Analysis window² are recorded here. The ³Result window² 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 ³Result window² 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: http://www.psychologie.uni-trier.de:8000/projects/gpower.html (II) Via FTP: Host: ftp.uni-trier.de User ID: anonymous Password: Your e-mail address. Directory: (a) if you want the Macintosh-version of G€Power: /pub/mac/local (b) if you want the MS-DOS-version of G€Power: /pub/pc/msdos Set the file type to binary (omitting to set the file type properly is a frequent error!). Start downloading (if you have to type in the file names, make sure you type them exactly as indicated below, this procedure is case-sensitive): (a) if you want the MACINTOSH-version of G€Power: GPower21.sit (b) if you want the MS-DOS-version of G€Power: GPower2i.exe (international version) or GPower2d.exe (German version) 7. Finish the FTP session: quit 8. Convert the compressed file to a standard Macintosh or DOS file: (a) Macintosh-version: Open the file GPower2.sit from a version of StuffIt or use the Stuffit Expander. (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) Address for correspondence ======================================================================== Please report bugs and suggestions to improve the program to: Axel Buchner FB I - Psychologie Universität Trier D-54286 Trier Germany E-mail: buchner@cogpsy.uni-trier.de ======================================================================== (5) 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.