Final answer:
A small effect size for two group means according to Cohen's d is around .20. Effect sizes range from small (.20) to medium (.50) to large (.80), with values in between signifying the magnitude of the difference.
Step-by-step explanation:
What would a small effect be for two group means? The answer is .20. In the context of comparing two group means, the effect size refers to the magnitude of the difference between them. Cohen's d is a commonly used measure to represent this. A small effect size is typically considered to be around .20, a medium effect size is around .50, and a large effect size would be about .80 according to Cohen's standards. Therefore, if we have an effect size of, for example, 0.384, this would represent a small effect size because it is between Cohen's value of 0.2 for a small effect and 0.5 for a medium effect size. This indicates that the difference between the means of the two groups is not very significant.
For instance, if the calculated effect size is 0.625, as per Cohen's criteria, this would be classified as a medium effect. The effect size is important as it provides context to the statistical significance of a study by telling us the practical significance of the differences found.