Which of the following describes the conventional thresholds for small, medium, and large effect sizes?

• A. Small = 0.2; Medium = 0.5; Large = 0.8
• B. Small = 0.1; Medium = 0.3; Large = 0.6
• C. Small = 0.5; Medium = 1.0; Large = 1.5
• D. Small = 0.2; Medium = 0.5; Large = 1.0

Answer: (A)

Interpreting how big an effect is often uses standardized benchmarks for the effect size, specifically Cohen’s d. The idea is to translate how many standard deviations apart the groups are into categories that help us grasp practical significance.

The standard set says a small effect is about 0.2, a medium around 0.5, and a large about 0.8. These values come from Cohen’s guidelines and are widely used as rough, intuitive benchmarks to describe the magnitude of differences or associations. They let researchers compare results across different studies and measures that might have different scales, because the effect size is standardized.

This set is the best match because it aligns with the widely cited convention for interpreting standardized mean differences. Other proposed sets don’t fit the common interpretation: they either push the “large” category to a much bigger value (like 1.0 or 1.5) or shrink the thresholds for small or medium to values that aren’t typically used in practice. So the 0.2 / 0.5 / 0.8 scheme remains the closest and most recognized standard for describing small, medium, and large effects.