What does 'effect size' measure in research?

Effect size is a statistical measure that quantifies the strength of a relationship or the magnitude of an effect observed in a study. It gives researchers insight into the practical significance of their findings, which is essential for understanding not just whether an effect exists (as determined by p-values), but how substantial or impactful that effect is in real-world terms. By evaluating effect size, researchers can gauge the importance of their results, which aids in comparing the strength of effects across different studies or measures.

The importance of effect size is highlighted when interpreting the results of statistical tests, as it provides a more comprehensive picture beyond mere statistical significance. This measurement facilitates better decision-making regarding the implications of research findings, such as in fields like psychology, education, and medicine, where understanding the degree of impact can guide practice and policy.

The other options represent different aspects of research methodology or design, but they do not capture the specific purpose of effect size. For instance, randomness of sample selection pertains to the sampling methods used and how well they represent the population, while the type of statistical test employed relates to the analysis conducted. Validity of research findings deals with the accuracy and credibility of the results, but does not directly address the measurement of effect magnitude.