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Original Articles

Effect Sizes

Why, When, and How to Use Them

Published Online:

The effect size (ES) is the magnitude of a study outcome or research finding, such as the strength of the relationship obtained between an independent variable and a dependent variable. Two types of ES indicators are sampled here: the difference-type and the correlational (or r-type). Both are well suited to situations in which there are two groups or two conditions, whereas the r-type, used in association with focused statistical procedures (contrasts), is also ideal in situations where there are more than two groups or conditions and there are predicted overall patterns to be evaluated. Also discussed are procedures for computing confidence intervals and null-counternull intervals as well as a systematic approach to comparing and combining competing predictions expressed in the form of contrast weights and ES indicators.