Alternative Approaches to Interpreting Effect size (2 of 2)


Keep in mind, however, that, just as with any measure of effect size, it is not possible to specify how big the effect must be in order to be important without considering the context in which the effect is going to be used. Just as with the measures of variance explained, there are cases for which an effect that has very small d' can have a substantial practical effect.

Graphical Methods
It is obviously difficult to summarize the size of an association with a single number. A better approach is to use an appropriate graphical display to indicate the size of an effect. When differences in the central tendency of two or more groups is at issue, then side by side box plots is an excellent way to portray the group differences. The differences between measures of central tendency and the amount of overlap among the groups is readily apparent. Scatterplots provide a useful way to portray the size of a relationship between two quantitative variables. Too often researchers rely solely on the magnitude of the correlation coefficient without viewing a scatterplot in their assessment of the size of a relationship.

Other Resources
There are many issues involving effect sizes that are beyond the scope of this book. The reader is referred to the book by Rosenthal, Rosnow, and Rubin for more information.

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