Professor John Hattie is, quietly, one of the most divisive figures in world education. Opinion is split between those who know of the New Zealander and praise him as the author of teaching's "holy grail", and those who know of him and accuse him of being an "egotistical, self- serving" academic, too keen to bask in the limelight.
For someone reputed to be unafraid to speak his mind, Hattie is remarkably blase about both points of view. He says the praise for his seminal work Visible Learning is "a bit ambitious", and as for the critics: "They are out there. I don't mind them."
So what is it that has made Hattie - often referred to the world's most important educational researcher - the focus of such global attention and controversy? In essence, the answer is his life's work. Over the past 20 years, he has compiled what is thought to be the largest educational research study of what works best in the classroom.
Speaking from Australia, where he has been director of the Melbourne Education Research Institute at the University of Melbourne since March 2011, Hattie initially comes across as relaxed and laid back. Sanguine, even. But start talking about teaching and learning and his drive and passion soon become evident.
Over the course of our conversation he attacks teacher training as "the most bankrupt institution I know", says educationalists spend too much time debating "things that don't matter" and claims too many teachers waste time looking for the "magic bullet".
It is this characteristic outspokenness that has galvanised many of Hattie's fiercest critics. But he doesn't mind: "One of the things we need to work on as academics is speaking out on the basis of what our research says. I'm prepared to call it as I see it until someone proves otherwise. Statements without evidence are just opinions - there are too many of those in education and that's what's got us into trouble. It's the interpretation of evidence that matters. I don't react to the personal attacks, only the evidential attacks."
Hattie's confidence in his statements is well founded. The result of his work, the groundbreaking 2008 book Visible Learning, is the largest ever collection of evidence-based research into what makes a difference to learning in schools.
It synthesised more than 800 meta-analyses (over 50,000 studies) encompassing the experiences of more than 80 million school-aged pupils across the English-speaking world. The book identified 136 classroom interventions and listed them in order of effectiveness, coming to the simple yet startling conclusion that the single most effective way to improve education is to raise the quality of the feedback pupils get and their interaction with teachers. It is pupils' ability to assess their own performance and to discuss how they can improve with the teacher that makes the most difference.
One of the most surprising findings was that many of the educational topics that dominate debates and headlines, such as class sizes, homework and choice of school, have little effect.
"The thing that really intrigues me is that the things at the bottom of the effects table still dominate our debates," says Hattie. "We like to talk about things that really don't matter, such as all the structural things and the way schools are set up."
This is very clearly a nod in the direction of the obsession in the UK with academisation and in the US with charter schools.
But the study, and Hattie himself, came in for most criticism for not delving into the impact on pupils of social factors such as home life and poverty. "Those things are critically important but I can't do much about them," he says. "Some people say because of that it's not valid and have therefore ignored what I have said. But what's more important is parental engagement and encouragement."
Following the evidence
Hattie started the work in the early 1990s after many years spent working in teacher training institutions in New Zealand, Canada and the US.
"Everybody I ever met in the education business told me what the answer was, and they were all different," he says. "It didn't make sense to me that everybody had evidence and could claim they were right. In the early days meta-analysis was a new concept, and I started wondering if I could use this method to put every educational intervention on a scale and measure them against each other.
"After the first few studies I started to find that 90 per cent of the things we do enhance achievement. So it answered the question - everything seemed to work. I didn't expect for a minute to find that everyone was right."
Over the next two decades he added more and more studies to the analysis until he published Visible Learning. Although the work would be more than enough to seal his legacy, Hattie admits that he cannot stop. In December last year, he published an updated version that added a further 14 interventions to the list, and he recently passed the 71,000th study.
"Some of the details change but the overall story doesn't," he says. "If anything, it's been confirmed."
Hattie's work has been widely praised, with an enthusiastic TES giving it the "holy grail" moniker, but he is typically self-effacing. "The reaction I have been getting over the past few years has been very positive; it's certainly increased the number of people who request things of me. But I'm just standing on the shoulders of others. I have a passion for trying to understand what makes a difference in education."
It is a field of work that Hattie admits he "fell into accidentally". Born in the port city of Timaru, New Zealand, in 1950, Hattie says he had a "productive kind of upbringing", attending the local boys' school and at one point dabbling in a career as a painter and decorator.
But his passion to "make a difference to the lives of people" led him to the University of Otago, where he gained a degree and a teaching diploma, becoming a tutor in education, a lecturer in educational tests and measurement and then a full-time teacher for a year. He gained an MA at Otago and then moved to Ontario, Canada, where he gained his PhD in 1981.
His remarkable career has seen him work at universities in his native New Zealand, Canada, the US and Hong Kong. Before taking up his present post in Melbourne he was professor of education at the University of Auckland.
He has authored or co-authored 12 books and more than 500 papers, has gained more than AUS$31 million worth of research grants and has advised governments across the world. Before his move to Australia, he was a member of an advisory group reporting to New Zealand's minister of education on standards in reading, writing and maths for primary school children. And in 2011 he was made an officer of the New Zealand Order of Merit in the Queen's Birthday Honours, for services to education.
After such an illustrious career it is easy to see where Hattie's self- confidence and outspoken nature come from. Perhaps not surprisingly, he still has plenty to say about education and teaching. He tells TES - quite clearly - that the biggest mistake teachers make is concentrating on testing and placing too much importance on test scores, something he accepts is an irony given his early background in testing and measuring.
"The most important thing you should worry about is growth for every student," he says. "Everyone deserves a year's growth for a year's input. Teachers should think more in terms of evaluating their impact. We have a mentality where we evaluate the students' impact. That's great, but the only way you do that is to have the teachers evaluating their impact on kids. If things aren't improving maybe they need to change.
"Too many teachers are constantly thinking that if they had more time, resources and space they could make a difference. For some teachers that could be true, but for most the last thing they need is more. They need different, and that's what they struggle with. It's simple: if your teaching practice is not having an effect on your students' performance, you must change."
He also thinks that teachers should work more collaboratively and talk about the things that matter. "Teachers aren't as good as they should be at knowing each other's impact and working with each other to change that," he says. "Very rarely do they talk about their teaching; it's all about curriculum, assessment and students.
"Too many teachers believe the essence of their profession is autonomy. We hardly ever get together and look at each other's teaching. That is a major hindrance to working collectively. I can't imagine many other professions where that happens."
Although he has worked with politicians of all persuasions and is unafraid to speak his mind, Hattie harbours no ambition to follow a political career. "I enjoy what I'm doing too much, and I don't enjoy the black-and- white nature politicians have to adopt," he says.
When he's not working he enjoys life in Australia with his wife Dr Janet Clinton, who is a senior lecturer, their three sons, Joel, Karl and Kieran, and their three dogs.
Despite being 62 and having a long and hugely successful career behind him and a string of letters after his name, Hattie has no intention of retiring any time soon. He is currently writing a new book on the processes of learning and plans to continue his meta-analysis. And, typically, he will continue to speak out on educational matters and defy the critics.
"I've always taken the view that I have a responsibility to speak out in public," he says. "I go into schools and have robust arguments and debates with teachers.
"There are critics out there. I don't mind them. Where it's criticism of my work I have no trouble with that. Some academics have picked up on what I should do better and that keeps me going. Some critics are personal and nasty, and some come up with an incredible set of conspiracy theories. I just ignore those. I still think I'm one of my biggest critics."