The Effect Size is the change in means divided by the standard deviation.
Take two Maths classes filled with children of equal ability. We are going to give them a test on a topic, teach them using two different methods, test them again and then calculate the Effect Size.
Class 1 – Gets an average mark of 50% at the start and an average mark of 60% at the end.
Class 1 gains 10%
Class 2 gets an average of 50% at the start and 70% at the end.
Class 2 gains 20%
Stop at this point and ask yourself which method of teaching do you think is better?
Now let’s calculate the Effect Sizes.
Class 1 has a standard deviation of 5% so its Effect Size is 10 divided by 5 = 2
Class 2 has a standard deviation of 20% so its Effect Size is 20/20 = 1
The Effect Size says that method 1 is better.
The Effect Size and the change in Means give different answers as to which method of teaching works best.