Question

why is effect size important to report after the results of a hypothesis test

Answer 1

Effect size is a statistical concept that measures the strength of the relationship between two variables on a numeric scale. For instance, if we have data on the height of men and women and we notice that, on average, men are taller than women, the difference between the height of men and the height of women is known as the effect size. The greater the effect size, the greater the height difference between men and women will be.

Reporting effect sizes provides all of the information available from “traditional” statistical reporting but also adds an element of the magnitude of the effect that simply cannot be ascertained from P values alone.