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How Much Difference Does It Make? Notes on Understanding, Using, and Calculating Effect Sizes for Schools

Ian Schagen and Edith Hodgen

Many schools have heard the term ‘effect size’ and are interested in finding out more. This paper explains what effect sizes are, how and why they might be calculated and applied, and some of the pitfalls in the process.

It is designed as a step-by-step guide for teachers and contains relevant examples and clear definitions of terms. It also contains tables showing effect sizes for PAT, STAR and AsTTle tests.

The paper is a collaboration between Dr Ian Schagen and Edith Hodgen. Dr Schagen is a former head of statistics at the National Foundation for Educational Research in the UK. In 2008 he came to work for a year with the Research Division of the Ministry of Education, as a Chief Research Analyst. Edith Hodgen is the manager of the Statistics and Data Management team at the New Zealand Council for Educational Research (NZCER).

The report is also on the Education Counts website:

Year published: 
Publication type: 
Research report
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Edith Hodgen
Ian Schagen