Sometimes scientists want to go beyond describing simple calculations like average heights or age in their populations, to understanding aspects of their populations that are more complex. Let us say we are not only interested in seeing how much sleep students get, but we want to know how much test scores will drop after losing a few hours of sleep. Effect sizes are values that estimate the magnitude of a phenomenon, or the degree to which one variable (like hours of sleep) impacts another variable (like test scores). For example, if getting only 3 hours of sleep lowers your test grade by a few points compared with when you get 9 hours of sleep, you might not "lose sleep" about losing sleep. While there is a difference in the score, that is not a large difference. However, if after losing 6 hours of sleep, you drop many points on a test, that could have a major impact on your grade. In this case, you would likely agree that the effect of losing sleep on your grades is an important one.

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Go to source: https://kids.frontiersin.org/article/10.3389/frym.2019.00118