From https://www.scribbr.com/statistics/effect-size/
Effect size tells you how meaningful the relationship between variables or the difference between groups is
Why use effect size?
- Practical significance (effect size) shows that the effect is large enough to be meaningful in the real world
- Statistical significance (p-value) only shows that an effect exists in a study
- Statistical significance alone can be misleading because influenced by sample size
- Increasing sample size makes it more likely to find statistically significant effect
- In contrast, effect size is independent of sample size
One measure of effect size is Pearson’s Correlation Coefficient
Calculate effect size before doing hypothesis tests:
- Knowing the expected effect size lets you figure out the minimum sample size you need for enough statistical power to detect an effect of that size