What is an effect size and why is it important in counseling research?

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Effect size is a standardized measure of how large or meaningful the treatment effect is. In counseling research, p-values can tell you whether an effect is unlikely to occur by chance, but they don’t tell you how big that effect is in real-world terms. The effect size quantifies the magnitude of the difference or the strength of the relationship, allowing you to compare results across studies and determine whether a finding has practical, clinical significance. This is crucial for deciding whether an intervention is worth implementing in practice, not just whether it reached statistical significance. Different forms of effect size (like Cohen’s d for mean differences, correlation r for relationships, or odds ratios for binary outcomes) let researchers express results in a way that’s comparable across studies and contexts. Reporting effect sizes, often with confidence intervals, also supports power planning for future research and enables meaningful meta-analyses. So, the concept being tested is a standardized measure of treatment impact that helps interpret practical significance beyond p-values.