# 19: Effect Size

- Page ID
- 2207

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- 19.1: Prelude to Effect Size
- Researchers often seek to learn more than whether the variable under investigation has an effect and/or the direction of the effect. This is particularly true for research that has practical applications.

- 19.2: Proportions
- Compute absolute risk reduction Compute relative risk reduction Compute number needed to treat

- 19.3: Difference Between Two Means
- When the units of a measurement scale are meaningful in their own right, then the difference between means is a good and easily interpretable measure of effect size.

- 19.4: Proportion of Variance Explained
- Effect sizes are often measured in terms of the proportion of variance explained by a variable. In this section, we discuss this way to measure effect size in both ANOVA designs and in correlational studies.