The two kinds of Effect Size

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


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.

Or we could give his relative speed, he’s travelling 10 mph faster than Ann.

It doesn’t matter which one we use as long as we all know which definition we’re talking about.

The problem comes when we start using the same words to mean two different things and start mixing them up.

When comparing Bob’s speed to Ann’s, a good actual speed would be anything over 40 mph, but a good relative speed would be anything over 0 mph.

Now, if I say that Cathy is going at a speed of 30 mph, is that an actual speed, in which case she’s going the slowest, or a relative speed in which case she’s travelling the fastest?

Hattie only mentions the two different types of Effect Size once, at the start of the book, but the way he talks later on “Everything works” and “We need to compare with 0.40” shows that the definition he is using is the first one, measuring actual improvement. However, has he made that distinction when he was choosing his studies? I suspect that he has not realised that the two different ways of measuring would produce very different answers and he has just thrown both types of study all in together.

For John Hattie, any Effect Size bigger than 0.40 would be ‘good’.

Now which version does the Education Endowment Foundation use?

In their recent Chatterbooks study they say





This and the fact that they use control groups show that they are using the second way of calculating the Effect Size, the relative way.

So, for the Education Endowment Foundation, anything better than zero would be ‘good’.

So, to sum up, the two major players using the ‘the Effect Size’, John Hattie and the Education Endowment Foundation, are actually using it to mean two completely different calculations, one actual and one relative. For one of them, anything above 0.40 would be ‘good’, the other anything above zero would be ‘good’.


Correspondence with John Hattie

Shortly after I started this blog, after I’d done a few posts, I wrote to John Hattie at the University of Melbourne pointing out some of my concerns. One of the things I pointed out was that he claimed the ‘Effect Size’ had units of standard deviation when it can be shown mathematically that it actually has no units (and it’s fine for it to have no units as long as you realise that).

In fairness to him, he wrote back quite a long letter taking each of my points in turn. When it came to my ‘the Effect size has no units’ point he said –

“It is not correct to claim that the Effect Size has no units, it does, from -infinity to +infinity but more normally between -3 and 3”

Now, up to this point, I couldn’t quite believe all that I’d found out about the Effect Size. I would say to myself ‘the Effect Size is wrong and you’re the only one who noticed. Yeah, right!’ I was constantly searching my mind thinking ‘You’ve missed something, what have you missed?’

When I read this statement from him my mouth just dropped open.

Not only does John Hattie not know what units the ‘Effect Size’ is measured in, he doesn’t even understand what units are. What he’s quoted are not the units but the typical magnitude of the ‘Effect Size’ as found in Education research. This is an error which throws doubt on John Hattie’s basic mathematical competence.

To give you an example of how big a gaffe this is, imagine you asked a Physics Teacher what the units of speed for a car are. ‘The units of speed of a car are between 0 and 70’ they answer. No, the units of speed are miles per hour (or kilometres per hour or metres per second). That is a significant mistake and you wouldn’t have a great deal of faith in the ability of the person who said it afterwards.

Is his letter, John Hattie also admonished me, saying that he had read my blog and felt I made too many remarks about him for him to leave a comment. He said –

“. . . in Academia the criticism is of ideas not people”

Which is fine except most people have no way to gauge whether or not a Mathematical argument is correct or not so they might need to rely on other questions to guide them, questions like –

– Do other people in relevant fields use this?

– What is the competence of the person using this?

Now these questions won’t give us the definite answer to the use of the ‘Effect Size’ but what they may do is indicate an area of concern that may be worthy of further investigation.

The answer to the first question is, Mathematicians and Scientists have never heard of the Effect Size, in fact only Psychologists and Education Researchers use it.

If you’re going to use Maths that Mathematicians don’t you’re either a genius, or you don’t know what you’re doing.

John Hattie is an Arts Graduate, who doesn’t understand what units are, nor the importance of getting them correct. I’ll leave you to ponder for yourself which he is.




Why the EEF report on Philosophy for children has no tests for statistical significance

I noticed a few days ago that people were expressing surprise on Twitter that the EEF report on Philosophy for children had no tests for statistical significance.

The problems with the statistics have been written about in greater detail here and also here.

OK. Maybe some people haven’t read my previous blogs or believed what I’ve said before (and it is quite shocking) so I will briefly explain again.

When Mathematicians invented modern-day Statistics in the 1930s, they needed a way to see if results from an experiment were a real effect or just randomness. (For example, I throw a coin 10 times and it comes up Heads 7 times, it’s probably just randomness. I throw a coin 100 times and it comes up Heads 70 times, it’s probably biased.) So, Mathematicians invented statistical significance and p values to separate randomness and real effects.

Now, along come some Psychologists. They said “Mathematicians are a bunch of idiots and they’re doing this all wrong, let’s invent our own way of doing things’. So they invented the Effect Size. Mathematicians and Scientists have continued using statistical significance and Psychologists and Educationalists have continued using the Effect Size. They have said repeatedly that Null Hypothesis Significance Testing (i.e. the way Mathematicians and Scientists do things) is wrong.


Statistics Hell

This kind of thing is repeated on numerous Social Science websites.

So, you’ve really got to understand this, it’s not a case of them choosing one technique over another.

The people who use the Effect Size think that statistical significance testing, i.e. the way Mathematicians and Scientists do things is wrong and they have invented their own way of doing Statistics. 

You’ve really got to grasp that to understand what I’ve been saying in my blogs.

How could all those people be wrong?

“How could thousands of Psychologists and Educationalists all make the same mistake? Entire fields doing incorrect Statistics. It’s simply not plausible.”

On Thursday night I read a piece called ‘The Art of being Right’ by Arthur Schopenhauer. Underneath I reproduce a few paragraphs from a section entitled ‘Appeal to Authority rather than Reason’.

“When we come to look into the matter, so-called universal opinion is the opinion of two or three people; and we should be persuaded of this if we could see the way in which it really arises.

We should find that it is two or three persons who, in the first instance, accepted it, or advanced it and maintained it; and of whom people were so good as to believe they had thoroughly tested it. Then a few other persons, persuaded beforehand that the first were men of the requisite capacity, also accepted the opinion. These, again, were trusted by many others, whose laziness suggested to them that it was better to believe at once, than to go through the troublesome task of testing the matter for themselves. Thus the number of these lazy and credulous adherents grew from day to day; for the opinion had no sooner obtained a fair measure of support than its further supporters attributed this to the fact that the opinion could only have obtained it by the cogency of its arguments. The remainder were then compelled to grant what was universally granted, so as not to pass for unruly persons who resisted opinions which everyone accepted. 

Since this is what happens, where is the value of the opinion even of a hundred millions? It is no more established than a historical fact reported by a hundred chroniclers who can be proved to have plagiarised it from one another; the opinion in the end being traceable to a single individual.”

Gene Glass should be the most famous man in Education. He is the person who changed the way the ‘Effect Size’ is used and spread its new use throughout Education. He became an Educational Psychologist in 1964. In the early Seventies he was receiving Psychotherapy and decided it had helped him so much that he wanted to prove to everyone that Psychotherapy worked. He’d learned about the ‘Effect Size’ from Jacob Cohen’s book ‘Statistical Power Analysis for the Behavioral Sciences’. (Jacob Cohen originally invented the ‘Effect Size’ and wrote a 500 page book explaining how to correctly use it to find the number of people you needed for your experiment.) Glass decided to completely change the way Jacob Cohen used the ‘Effect Size’, throw away the carefully constructed statistical look-up tables and use it for a completely different reason, sticking results together. While he was doing this, Glass was also elected as the President of the American Educational Research Association. He used his Presidential address to 1,500 educational researchers to announce his new method of putting results together using the new way of using the ‘Effect Size’. How many of those researchers would have thought that there was any element of doubt in what this eminent man was telling them at this prestigious occasion? How many of them would have had the necessary expertise to tell if it was correct or not? Glass wrote a 2 page pamphlet justifying his new way (this has a few sketches on it as proof) and published an article with his wife, Mary Lee Smith, in ‘American Psychologist’. Psychologists and Educationalists all started to copy him and the new method spread throughout Psychology and Education.

So, imagine all the children of the world, underneath them, supporting them are the teachers from all the different countries, underneath them is the whole of education research and all of this, resting on his shoulders, is just one man, Gene Glass. Given that Mathematicians have never taken the remotest bit of interest in the ‘Effect Size’, are we absolutely sure he’s correct?

Now the writer of the EEF report on Philosophy says that the way Mathematicians and Scientists do Statistics is all wrong

The writer of the EEF report on Philosophy, Professor Stephen Gorad, has now openly admitted that he thinks that the way that Mathematicians and Scientists do Statistics is wrong and should be banned.

The significance test is how Mathematicians and Scientists do Statistics.

Psychologists invented the Effect Size as the “New Statistics” to replace it. It is unknown to Mathematicians.


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When the Physicists at the Large Hadron Collider were looking for the Higgs Boson particle, to be sure they had really found it they used a significance test, called the five sigma test.


So, on one side of the argument we have the people who found the Higgs Boson, the other, Stephen Gorard. The decision is yours.

The Chinese Way

The Government announced today that they are spending £11 million to have 32 hub schools, bringing Chinese teachers to the UK and sending our teachers to China so we can learn how they do so well compared to us in Mathematics.

Yay, there’s a magic bullet that means the children can achieve without hard work or behaving themselves? I know we’ve been fooled many, many times before but this time it’s really true.

But wait, what’s this?  If we look at the report released by Parliament – ‘Underachievement of White Working Class children



We see that the poorest Chinese children beat the richest non-Chinese children in this country.

It’s almost as if there is something else going on and the Chinese don’t have a magic way of teaching Maths at all.

The answer is of course that the Chinese have a culture of hard work and respect for education as typified in the ‘Battle hymn of the Tiger Mother‘ book by Amy Chua. They bring this with them when they move countries enabling them to come top in our country as well.

Maybe we need to be worrying more about copying the attitudes and culture of the Chinese parents and children towards Education and a little less trying to copy a mythical, magic, Chinese way of teaching.