Effect size

Researchers often seek to learn more than whether the variable under investigation has an effect and/or the direction of the effect. This is particularly true for research that has practical applications. For example, an investigation of the efficacy of a pain-relief drug would seek to determine the extent of the relief and not merely whether there was any relief.

- David M Lane: Online Statistics Education

Effect sizes may be measured in relative or absolute terms. In relative effect sizes, two groups are directly compared with each other, and for absolute effect sizes, a larger absolute value always indicates a stronger effect. Some frequently used terms that signal “effect size here!” are, for example:

• correlation
• odds ratio
• risk ratio (a.k.a. relative risk)
• hazard ratio
• mean (average) difference

Learn more about effect sizes from

• Online Statistics Education: Effect size