Final answer:
The index commonly used to measure Effect Size in group-level clinical significance is Cohen's d, which compares the differences between two means against established benchmarks for small, medium, or large effect sizes.
Step-by-step explanation:
The index commonly used to measure the Effect Size (ES) in group-level clinical significance is Cohen's d. Cohen's d is a measure of effect size that evaluates the differences between two group means. It is named after the U.S. statistician Jacob Cohen. The value is calculated by taking the difference between two means divided by the pooled standard deviation. The effect size is then compared against Cohen's benchmarks, which characterize effect sizes as small, medium, or large.
For example, an effect size of 0.834 would be considered large, as it exceeds Cohen's threshold of 0.8 for a large effect. Likewise, an effect size of 0.625 would be considered medium because it falls between the thresholds for medium (0.5) and large (0.8) effect sizes as defined by Cohen's standards.