Effect size calculation

Effect Size Calculation and Graphing

Given the mean, N, and standard deviation of two normal distributions, this web-based program will estimate Cohen's Effect Size (d) and the Common Language Effect Size. It will also produce a graph of the two overlapping normal distributions, as well as a graph of the proportion of each group at each level on the measured trait. Your web browser must be set to accept Javascript (if it is not, in the grey box below "Enter the name of group #1" will not appear).

Note: We have discovered a bug in the program when using data with groups with large mean differences. Until this bug is fixed, if there is a large difference in the means of your groups, we suggest that you get another calculation to confirm the effect size for your data.

Enter the data requested below (scroll down). You will need to enter the number of subjects and the mean and standard deviation, for each distribution, as well the names you would like to give to each distribution group. After you have entered the data, a graph will appear in the blank area below.

EXAMPLE

To calculate the effect size for the sex difference in height, enter the following data (which are for adult Americans, in inches). Click on the button labeled "Enter" after you've entered each value.

Group #1: Males

Enter the name of the group: Males.
Enter the mean height: 69.7.
Enter the standard deviation of the distribution of male height: 2.6
For the number of subjects, you may enter 1,000.

Group #2: Females

Enter the name of the group: Females.
Enter the mean height: 64.3.
Enter the standard deviation of female height: 2.8.
For the number of subjects, you may enter 1,000.

Then enter for the title: The Sex Difference in Height

Saving the graph to a file: The applet does not support saving the graph to a file. However, you may use a screen capture program to save the graph to a file. To download a screen capture program, go do a shareware/freeware download site (e.g., www.cnet.com or www.download.com) and use the search terms "screen capture." A good program is Quick Screen Capture.
If you would like to run the program again, press the keyboard keys Ctrl and Shift at the same time, while also clicking (on your screen) the "Reload" (or "Refresh") icon on your web browser. This will clear the screen so you can enter a new set of data.

Known Issues:

Scrolling the window after the graph has been drawn will sometimes cause errors in the visual presentation of the graph. Avoid this by positioning the data entry field near the bottom of your screen, so that you do not have to scroll to see the entire graphic after it appears.
The graph of the proportion of members of each group at various levels is not smoothed correctly.

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