What is the effect size that indicates a large effect?

In the context of effect sizes, a value of 0.8 is widely recognized as indicative of a large effect according to Cohen's benchmarks. Effect size is a quantitative measure of the magnitude of a phenomenon. Cohen suggested specific thresholds for interpreting effect sizes: a small effect is around 0.2, a medium effect is around 0.5, and a large effect is around 0.8.

The significance of effect size lies in its ability to provide a clearer understanding of the practical implications of research findings. A large effect size (0.8) suggests that the observed phenomenon has a meaningful impact and is likely to be of importance in practice and theory, making it a more compelling result in behavioral sciences, including clinical psychology.