The two kinds of Effect Size

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At the start of ‘Visible Learning’, John Hattie talks about the two different ways to calculate the Effect Size.

Effect Size = (Mean of group at end of intervention – Mean of group at start of intervention) / Standard Deviation

and

Effect Size = (Mean of intervention group – Mean of control group) / Standard deviation

Now, obviously, real Mathematical things don’t have two different definitions because of the confusion this causes as we shall see.

The problem is that the first definition measures actual improvement whereas the second measures relative improvement.

To give an example, imagine Ann and Bob are both travelling to an education conference. They set off at the same time, driving their cars down the motorway.

We know that Ann drives at an average speed of 40 mph. How can we tell if Bob will get there first?

We could give his actual speed of 50 mph.

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