An effect size that expresses the difference between two means in standard deviation units.

An effect size that expresses the difference between two means in standard deviation units.

• A. Cohen's d
• B. Eta squared
• C. Pearson r
• D. Hedge's g

Answer: (A)

The difference between two means expressed in standard deviation units is the standardized mean difference. The classic measure for this is Cohen's d. It calculates how far apart the means are in units of the pooled standard deviation, giving a unitless value that shows how many standard deviations separate the groups. This standardization makes results easy to compare across studies and scales. For interpretation, values around 0.2 are small, about 0.5 are medium, and around 0.8 or higher are large. Hedge's g is a very similar measure that adds a small-sample correction, so it behaves like Cohen's d but is preferred with small samples. Eta squared describes the proportion of variance explained, and Pearson r reflects the strength of a linear relationship, not a mean difference in SD units. So Cohen's d is the best answer.

The difference between two means expressed in standard deviation units is the standardized mean difference. The classic measure for this is Cohen's d. It calculates how far apart the means are in units of the pooled standard deviation, giving a unitless value that shows how many standard deviations separate the groups. This standardization makes results easy to compare across studies and scales. For interpretation, values around 0.2 are small, about 0.5 are medium, and around 0.8 or higher are large. Hedge's g is a very similar measure that adds a small-sample correction, so it behaves like Cohen's d but is preferred with small samples. Eta squared describes the proportion of variance explained, and Pearson r reflects the strength of a linear relationship, not a mean difference in SD units. So Cohen's d is the best answer.