Effect size is a population parameter. It describes the population. In a sample, the ideal one would be zero if there are no differences between the two groups. It would increase either in a positive or a negative direction as either p1 becomes much better than p2 in the positive direction, or p2 is much better than p1 in the negative direction.
If I randomly take a subject from the p1 population and I randomly take a subject from the p2 population, what’s the probability that the p1 subject is better than the p2 minus the probability that p2 is better than p1? If that number is 0, that difference between them, and that’s what this idea is, that number is 0, that’s going to be because there really isn’t any overall difference between the two populations.
To learn more from Dr. Helena Kraemer listen to the podcast episode below.
Effect Size Part 1
Effect Size Part 2