What does 'effect size' measure in research?

Effect size is a quantitative measure that expresses the strength of a relationship or the magnitude of an effect observed in research. This metric is crucial because it goes beyond just indicating whether a result is statistically significant or not; it helps researchers understand the practical significance of their findings.

In psychology, social sciences, and many other fields, effect size provides a way to assess how strongly variables are related or the impact of an intervention. For example, if a study finds that a new teaching method significantly improves test scores, the effect size would indicate how much improvement is expected as a result of that method, providing insights into its real-world applicability.

The other options do not accurately reflect what effect size measures. The randomness of sample selection pertains to sampling methods and bias, while the type of statistical test relates to the analysis methods used in a study. Validity, on the other hand, addresses whether the research accurately measures what it is intended to measure. Thus, the option that represents effect size appropriately is the measure of the strength of a relationship between two variables.