The Effect Size and the change in Means can give different answers as to which method of teaching works best

The Effect Size is the change in means divided by the standard deviation.

Example

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.