An effect is the result of something. It is an outcome, a result, a reaction, a change in Y brought about by a change in X.

What is an effect size? An effect size refers to the magnitude of the result as it occurs, or would be found, in nature, or in a population. Although effects can be observed in the artificial setting of a laboratory or a sample, effect sizes exist in the real world.

Effect sizes are ubiquitous. You can find them in newspapers, college brochures, shop windows, Facebook ads, product packaging, church newsletters, blogs, tweets, TV commercials—just about anywhere. Here are some everyday examples of effect size:

• lose 20 pounds in four weeks on the South Beach diet
• learn how to speak Swahili in six months
• make $2,300 a day working from home
• improve test performance through meditation
• fast-track your career with an MBA
• list your property with us and sell your home within a week
• read this book and improve your publication prospects

When researchers estimate effect sizes by observing representative samples, they generate an effect size estimate. This estimate is usually expressed in the form of an effect size index.

For a good introduction to effect sizes – how to report them, how to interpret them – check out my book Effect Size Matters…