How much difference does it make? Notes on understanding, using, and calculating effect sizes for schools

Publication Details

A good way of presenting differences between groups or changes over time in test scores or other measures is by ‘effect sizes’, which allow us to compare things happening in different classes, schools or subjects regardless of how they are measured. This booklet is designed to help school staff to understand and use effect sizes, and includes handy tips and warnings as well as useful tables to calculate effect size values from change scores on standardised tests.

Author(s): Ian Schagen, Research Division, Ministry of Education and Edith Hodgen, New Zealand Council for Educational Research.

Date Published: March 2009

Please consider the environment before printing the contents of this report.

This report is available as a download (please refer to the 'Downloads' inset box).  To view the individual chapters please refer to the 'Sections' inset box.


Thanks are due to John Hattie of Auckland University, and colleagues in the Research Division at the Ministry of Education and at NZCER who have commented on the content of this paper at various times. However, ultimately the opinions expressed in this paper belong to the authors alone and are not to be construed as official policy from either organisation.

Ian Schagen
Edith Hodgen
March 2009

Dr Ian Schagen was Head of Statistics at the National Foundation for Educational Research in the UK until April 2008, when he came to New Zealand for a year, working 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 NZCER.

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