Drawing Graphs with G*Power

All parameters which G*Power needs in order to draw a graph in the graph window can be specified in the dialog box shown below.

The controls in the dialog can be decomposed into five different groups: Header Axes Input values Calculation mode Plot Parameters

1. Header

The two lines of text which you see in the picture below cannot be changed. They just show some of the inputs you made in the Analysis Window.

2. The Axis

In this section you select the type of data you want to plot as a function of another type of data. Just select one field in each of the pop-up menus. For instance, if you want to see the effect size as a function of the sample size, select effect size in the upper (y-axis) and sample size in the lower (x-axis) pop-up menu.

The pop-up menus for the x-axis and the y-axis let you plot any of the central parameters of a power analysis as a function of any other parameter. 'Start' and 'Stop' values determine the plot range. The function will be plotted in the interval [Start,Stop]. The scaling of the y-axis is automatically adjusted as a function of the size of the graph window. The increment parameter determines the "resolution" of the output. This means that the result of the function will be calculated at equidistant points. A lower increment value produces a less "grainy" output, but it takes more time to do the calculations.

3. Input values

Two of the four parameters of a power analysis must remain fixed in order to plot the third parameter as a function of the fourth. This is where you specify the values of the two fixed parameters if their values need to be different from the ones you specified in the input value section of the Analysis Window.

For further information look here.

We provide further information to these parameters here: Alpha error probability Power and beta error probability Effect size Total sample size

4. Calculation mode

G*Power knows two calculation modes, one optimized for speed and another mode which yields for high accuracy. For more information about the two different modes, look here.   Please do not use Speed mode calculations for your final power analyses!

5. Plot Parameters

You have several options to determine the form of a graph. Note that all drawing options can be turned off and on independently.

You can determine the appearance of your graphical output

For additional information check: Grid Plot values Draw a line Protocol table

If the Grid option is on, G*Power draws a grid to facilitate the inspection of the output.   If the Plot Values option is on, G*Power plots the y-values (the function results) above the data points. If numbers overlap, try to resize the graph window (or increment the increment parameter. If that does not help, select protocol table to see the numerical values of all points that are plotted in the graph.   If the Draw Line option is on, G*Power interpolates all data points with straight lines.   If the Protocol table option is on, the numerical values of all data points are written to the Result Window. You can edit, save, or export the data to an other program (i.e. your favorite word processor).

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