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Commentary
Evidence does not supply us with rules for action but only with hypotheses for intelligent problem solving, and for making inquiries about our ends in education. (John Dewey, quoted in Hattie, 2008, p. 147)
INTRODUCTION
This book by Professor John Hattie of Auckland University is the result of decades of careful research. He has synthesised some 800 meta-analyses comprising more than 50,000 studies and involving some 146,000 'effect sizes'. The announcement of the book has already led to a good deal of discussion both in New Zealand and overseas and seems to have captured the attention of policy-makers. It is, therefore, important that members of the educational research community pay John Hattie the courtesy of subjecting his conclusions to critical scrutiny in a spirit of mutual truth-seeking to ensure that: (1) discussions are based on a careful reading of the book, rather than on half-baked 'reactions' in the popular media; (2) the caveats which Hattie himself sets out are carefully noted so that decisions are not made in opposition to the message of this book, and (3) the findings are not 'appropriated' by political and ideological interests and used in ways which the data do not substantiate.
THE METHODOLOGY UNDERLYING THE BOOK
Hattie derives his results from working on a large sample of research studies. His method involves a synthesis of a large number of meta-analyses of studies about education variables. A meta-analysis is a statistical technique for amalgamating, summarising and reviewing primary research. It combines the results of various studies which address a set of research hypotheses. It is used in many branches of knowledge such as medicine, psychotherapy, business and education. All the findings in this book derive from John Hattie's synthesis of 800 meta-analyses of more than 50,000 quantitative studies of variables affecting the achievement of students.
A major aim is to determine effect sizes. From looking at a large number of research studies it is relatively easy to determine that there are certain effects: for example, overall, drug A is more successful in lowering blood pressure than drug B. But the key question is, "How much more successful?" Effect size is a way of answering this question. It involves comparing the mean scores of the two variables and...