What does effect size measure?

Effect size measures the magnitude of a relationship between two or more variables, providing a quantitative assessment of the strength and direction of this relationship. It goes beyond merely indicating whether an effect exists (as statistical significance does) and instead offers insight into how meaningful that effect is in practical terms.

For example, in clinical psychology, understanding effect size can help determine the impact of an intervention by illustrating how much improvement is observed in a treatment group compared to a control group. This information is crucial when evaluating the effectiveness of different therapeutic approaches, as it helps clinicians and researchers understand not just if a treatment works, but how effective it is in real-world settings.

While other concepts such as the probability of a Type 1 error, variance within a single variable, and standard deviation relate to statistical analysis, they do not provide information about the strength or relevance of a relationship between variables in the same way that effect size does.