In an increasingly data-driven world, it is more important than ever for students as well as professionals to better understand basic statistical concepts. 100 Questions (and Answers) About Statistics addresses the essential questions that students ask about statistics in a concise and accessible way. It is perfect for instructors, students, and practitioners as a supplement to more comprehensive materials, or as a desk reference with quick answers to the most frequently asked questions.

# What Is Effect Size, and Why Is It Important?

### What Is Effect Size, and Why Is It Important?

You already know that there is an important difference between statistical significance and the meaningfulness of the results of an analysis (see question #78). But there is another very useful way to judge the value of a statistical finding, and that’s through the use of effect size.

Effect size is a measure of the magnitude (not necessarily the absolute size) of a statistical finding. In other words, there may indeed be a difference between the averages of two samples, but the effect size might be very low and, therefore, the differences relatively meaningless. On the other hand, there may be a small absolute difference between the averages of two groups, but the effect size could be ...