A Description of the G*Power User Interface

The following section introduces the basics of the G*Power user interface and it describes how to perform power calculations. Although it contains some examples, first time users should start with the tutorial. Also please note that we use the Macintosh version of G*Power to explain the interface. However, the MS-DOS version is very similar so that MS-DOS/Windows users can actually use these explanations as well.

Main Windows

G*Power has three main windows which are displayed on this page.

1. Analysis Window

The Analysis window is used to specify the information necessary for a particular power analysis. More information about the items of the main window and how to do power analyses can be found in a separate section.

2. Result Window

If the result window is open, all parameters of all power analyses that you calculate are recorded here. Also, 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.

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 window to a G*Power text file using the Save or Save As commands from the File Menu (double-clicking such a text file launches G*Power, but the file may also be opened from within your favorite word processor).

3. Graph Window

The graph window can be used to display one parameter (e.g., the total sample size) as a function of another parameter (e.g., the power) over a range of values. G*Power allows you to plot any of the parameters (effect size, sample size, alpha and beta) as a function of any other parameter. Find out more about the generating of graphs with G*Power in the next section.

The header of the graph always specifies the type of test and the fixed values. The note below the graph tells you which calculation mode you used. If it is speed mode, do not trust the values too much. You can copy the graph (using Copy Graph from the Edit menu) and past it into your word processor or drawing application for saving and editing the graph information. Inside G*Power you cannot edit or save a graph (this restriction helped to keep the program small and handy). The scaling of the variables depends on the size of the graph window. Reseizing this window will cause a more or less fine-grained scaling.

Please report suggestions for improvements to Axel Buchner, Franz Faul, or Edgar Erdfelder.