What is Cohen's d?

What is Cohen's d?

• A. A measure of effect size that indicates the standardized difference between two means.
• B. The probability of a Type I error.
• C. The reliability of a test across time.
• D. The correlation between two variables.

Answer: (A)

Cohen's d is a measure of effect size that indicates the standardized difference between two means. It’s calculated by subtracting one group mean from the other and dividing by the pooled standard deviation, giving a unitless value that lets you compare the size of differences across different scales. Interpreting the magnitude, roughly 0.2 is a small effect, around 0.5 a medium effect, and about 0.8 a large effect, though context matters. This metric is useful alongside p-values to convey practical significance in experiments and meta-analyses. It differs from the probability of a Type I error (alpha), from test-retest reliability, and from the correlation between two variables, which are other statistical concepts.

Cohen's d is a measure of effect size that indicates the standardized difference between two means. It’s calculated by subtracting one group mean from the other and dividing by the pooled standard deviation, giving a unitless value that lets you compare the size of differences across different scales. Interpreting the magnitude, roughly 0.2 is a small effect, around 0.5 a medium effect, and about 0.8 a large effect, though context matters. This metric is useful alongside p-values to convey practical significance in experiments and meta-analyses. It differs from the probability of a Type I error (alpha), from test-retest reliability, and from the correlation between two variables, which are other statistical concepts.