What does the effect size quantify in a study?

• A. Magnitude of difference between groups independent of sample size
• B. Statistical significance
• C. Confidence interval width
• D. Sample size needed for significance

Answer: (A)

Effect size measures how large the difference or relationship is, independent of how many participants were studied. It’s a standardized gauge of magnitude, so you can compare effects across studies or measures. For example, Cohen’s d for a difference between two groups uses the difference in means divided by the pooled standard deviation, giving a value (like 0.5) that reflects a medium-sized difference regardless of sample size. This matters because statistical significance (the p-value) depends on both the size of the effect and how many data points you have: very large samples can yield significant results for tiny effects, while small samples may miss moderate effects. Confidence interval width reflects precision, which is influenced by sample size and variability, not the magnitude of the effect itself. Planning a study involves power analysis to determine the needed sample size to detect a given effect size with a chosen level of significance.