Hattie constantly proclaims 

'it is the interpretations that are critical, rather than data itself.' 

His key interpretations are that an effect size d = 0.40 is the 'hinge' point' for identifying what is and what is not effective, and also is equivalent to advancing a child’s achievement by 1 year (VL 2012 summary p3).

His rankings reflect this interpretation. However, the emergence of nationwide USA benchmark effect sizes of student progression from one year to the next raises significant doubt on all of Hattie's interpretations and inferences - see summary table here.

There are also major issues with his methodology: he relies too heavily on correlation studies, many studies do not measure changes in achievement and meta-analyses do not consistently get the same results. Also, the use of averaging across disparate results is questionable - see validity and reliability.

In addition, there are many errors, from misinterpreting studies to calculation mistakes and excessive inference.

This indicates that 'teaching' like any other 'people' science, is complicated and full of nuances. What works with one student or class or in one context may not work in another (the emerging field of Contextual Psychology also analyses this complexity). As Prof Jill Blackmore states,
'You may have a teacher in one class and they do really well and then you put them in another classroom or in a different school and they are not that crash hot. It’s all about the class …, it so complex… It should be about valuing teachers as professionals and giving them the capacity to make professional judgements' (p56).

Whilst it may be simpler and easier to see teaching as a set of discreet influences, the evidence shows that these influences interact in ways in which no-one, as yet, can quantify. It is the combining of influences in a complex way that defines the 'art' of teaching. 

The notion that an influence is the 'best' or a 'disaster' (as Hattie presented in the 2005 ACER conference) is dangerous and misleading - not the least of which are the political ramifications.

Given the definitive statements that Hattie and his supporters make, it is important to bear in mind the critique by statisticians such as Professor Arne Kare Topphol:

'My criticism of erroneous use of statistical methods will thus probably not affect Hattie’s scientific conclusions. However, my point is, it undermines the credibility of the calculations and it supports my conclusion and the appeal I give at the end of my article; when using statistics one should be accurate, honest, thorough in quality control and not go beyond one's qualifications.

My main concern in this article is thus to call for care and thoroughness when using statistics. The credibility of educational research relies heavily on the fact that we can trust its use of statistics. In my opinion, Hattie’s book is an example that shows that we unfortunately cannot always have this trust.'

The people and websites that proclaim, using Hattie's inferences, that class size effects, or diet effects or ability grouping, etc, are myths need to be challenged - have you read the studies? Are they measuring achievement? How are they doing this? Is the study a true experiment or a correlation? Are there discrepancies or confounds? What is the age of the students? What do the researcher's conclude? Is this consistent with Hattie's summary?

More caution is in order, as educational guru Andy Hargreaves says in his book 'The Fourth Way':

'Even with all the best evidence in the world’s top hospitals, medicine remains 'an imperfect science', an enterprise of constantly changing knowledge, uncertain information, fallible individuals' (p34).

'It doesn’t treat statistical evidence as gospel truth leading to unarguable commandments, but rather as a process of interaction in a professional learning community where everyone wants to improve. Players on this professional team see data as their companion- not as their commander' (p37).

Hattie seems to denigrate widespread teacher experience and his campaign "Statements without evidence are just opinions" (borrowed from Edwards Deming). This seems to raise the status of his research and belittle teacher experience further. However, teacher experience was seen as 'action research' or 'case study'. The Medical Profession still relies heavily on case study - usually one doctor with a patient under a particular set of circumstances. In fact, one of the studies that Hattie cites heavily, the New Zealand government funded extensive study by  Alton-Lee (2003) promotes case study in conjunction with other research:

'It is with the rich detail of case studies that the complexity of the learning processes and impact of effective pedagogy can be traced in context, in ways that teachers can understand implications for their practice' (p13).

After reading much of the research Hattie cites I am not convinced that it is possible to separate influences in the precise surgical way in which Hattie infers. Most of his researchers also have doubts about this. I find Hattie's approach misses much of the complex interactions of many influences in the classroom.

Math's Teaching Revolution:

I have been following Professor Jo Boaler who is developing many strategies and resources to help improve math's teaching. Most of her strategies, e.g. problem-based learning, Computer-assisted instruction, Creativity, Simulations, Inductive teaching, Inquiry-based teaching,Visual/audio-visual methods, Web-based learning, Within-class grouping, Student control over learning, kinesthetic awareness and Teacher subject matter knowledge, are strategies which fall below Hattie's "worthwhile" point of d = 0.40. Click here for an example of her use of visualisation and kinesthetic awareness in Math's teaching.

Other, initiatives similar to Professor Boaler, which contradict Hattie's interpretations are the Maths 300 projectThe Maths Task Centre, and Prof James Tanton.

Also, helpful are Prof Dylan Wiliam's work on formative assessment and the U.S. National Center for Education Research's Teacher Practice Guide which is based on higher quality studies than Hattie's work. Both are free and easy to apply - see Other Researchers.

We also need to spend more time promoting and congratulating good teaching, like Maths Guru, Prof Steven Strogatz tweet about an Australian maths teacher.

In conclusion, there is widespread doubt about the effect sizes, rankings and inferences in Visible Learning.

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