# John Hattie admits that half of the Statistics in Visible Learning are wrong (Part 2)

In an earlier post we discovered that John Hattie had admitted (quietly) that half of the Statistics in Visible Learning were incorrect. John Hattie uses two statistics in the book, the ‘Effect Size’ and the ‘CLE’. All of the CLEs are wrong through-out the book.

Now, I didn’t really know why they were wrong, I thought, maybe he was using a computer program to calculate them and it had been set up incorrectly. I didn’t know. Until I received this comment from Per-Daniel Liljegren. He was giving a seminar on Visible Learning for some teachers in Sweden and didn’t understand some of what he’d found, so, he wrote to Debra Masters, Director of Visible Learning Plus, asking for help.

“Now, when preparing the very first seminar, I was very puzzled over the CLEs in Hattie’s Visible Learning. It seems to me that most of the CLEs are simply the Effect Size, d, divided by the square root of 2.

Should not the CLE be some integral from -infinity to d divided by the square root of 2?”

And if you grab your copy of Visible Learning and check, he’s right! The CLEs are just the Effect Size divided by the square root of 2.

If we look at this article that tells us how to calculate the CLE – How to calculate the Common Language Effect Size Statistic – we see that dividing by the square root of 2 actually finds the z value. This should then have been converted into the probability using standard Normal distribution tables, a very basic statistical technique that we teach to Year 12s in S1 A Level.

Throughout the book, Visible Learning, John Hattie has calculated the z values and used them as his CLEs when he should have converted them into probabilities.

Three very worrying things about all this –

1.   John Hattie doesn’t know the difference between z values and the probabilities you get from z values. Really, really, basic stuff.

2.   John Hattie knows about this mistake but has chosen not to publicise it. This could mean that many teachers are still relying on it to instruct their teaching.

2.   No-one picked up on it for years, despite the fact the CLE is meant to be a probability. So, throughout the book he is saying that probability can be negative or more than 100%. So, who is checking John Hattie’s work? Because the academic educational establishment doesn’t appear to be.

Again we are left with two options to choose from

1.   John Hattie is a genius who is doing things that even Mathematicians don’t understand.

2.   John Hattie is a well meaning man with a Social Sciences degree who has made a mistake in using statistical techniques he didn’t realise were unknown to Mathematicians and incorrect.

The choice is yours.

# John Hattie admits that half of the Statistics in Visible Learning are wrong

At the researchED conference in September 2013, Professor Robert Coe, Professor of Education at Durham University, said that John Hattie’s book, ‘Visible Learning’,  is “riddled with errors”. But what are some of those errors?

The biggest mistake Hattie makes is with the CLE statistic that he uses throughout the book. In ‘Visible Learning, Hattie only uses two statistics, the ‘Effect Size’ and the CLE (neither of which Mathematicians use).

The CLE is meant to be a probability, yet Hattie has it at values between -49% and 219%. Now a probability can’t be negative or more than 100% as any Year 7 will tell you.

This was first spotted and pointed out to him by Arne Kare Topphol, an Associate Professor at the University of Volda and his class who sent Hattie an email.

In his first reply –  here , Hattie completely misses the point about probability being negative and claims he actually used a different version of the CLE than the one he actually referenced (by McGraw and Wong). This makes his academic referencing, hmm, the word I’m going to use here is ‘interesting’.

In his second reply –  here , Hattie reluctantly acknowledges that the CLE has in fact been calculated incorrectly throughout the book but brushes it off as no big deal that out of two statistics in the book he has calculated one incorrectly.

There are several worrying aspects to this –

Firstly, it took 3 years for the mistake to be noticed, and it’s not as though it’s a subtle statistical error that only a Mathematician would spot, he has probability as negative for goodness sake. Presumably, the entire Educational Research community read the book when it came out and they all completely missed it. So, the question must be asked, who is checking John Hattie’s work? As a Bachelor of Arts is he capable of spotting Mathematical errors himself?

In Mathematics, new or unproven work is handed over to unbiased judges who go through it with a fine toothcomb before it is considered to have the stamp of approval of the Mathematical community. Who is performing this function for the Educational community?

Secondly, despite the fact that John Hattie has presumably known about this error since last year there has been no publicity telling people that part of the book is wrong and should not be used. Surely he could have found time between flying round the world to his many Visible Learning conferences to squeeze in a quick announcement.

As one of the letter writer’s stepfather, a Professor of Statistics said

“People who don’t know that Probability can’t be negative, shouldn’t write books on Statistics”

Sources –

Book review – Visible Learning by @twistedsq

Can we trust educational research? – (“Visible Learning”: Problems with the evidence)

EDIT – Since this post we have also discovered why the CLEs are all wrong and the reason is shocking. Read about it here – John Hattie admits that half of the Statistics in Visible Learning are wrong (Part 2).