5 Effect Size Estimation and Confidence Intervals

Research Methods in Psychology


  1. Fiona Fidler PhD1,
  2. Geoff Cumming DPhil2

Published Online: 26 SEP 2012

DOI: 10.1002/9781118133880.hop202005

Handbook of Psychology, Second Edition

Handbook of Psychology, Second Edition

How to Cite

Fidler, F. and Cumming, G. 2012. Effect Size Estimation and Confidence Intervals. Handbook of Psychology, Second Edition. 2:I:5.

Author Information

  1. 1

    The University of Melbourne, Australian Centre of Excellence for Risk Analysis (ACERA), Victoria, Australia

  2. 2

    La Trobe University, School of Psychological Science, Victoria, Australia

Publication History

  1. Published Online: 26 SEP 2012


We describe a six-step estimation framework for research that starts with the formulation of research goals in terms of “How much?” questions. Such questions are best answered by effect size (ES) estimates and confidence intervals (CIs) calculated from data, where the ESs estimates and CIs are point and interval estimates of population parameters. These estimates usually provide the best basis for the interpretation of research findings. Such an estimation approach includes use of precision-measured by CI width-for the planning of research, and extends naturally to meta-analysis. We explain why an estimation approach is usually highly informative, and much better than traditional null hypothesis significance testing (NHST). We describe a range of ES measures and explain how to calculate CIs for these measures. We emphasize choice of ESs that most closely correspond to the research questions; construction of figures that support good understanding of ESs and CIs; and the importance of meta-analytic thinking-the consideration always of the current study in the context of past and possible future studies that address similar questions. Psychology can improve its research greatly by adopting estimation much more widely.


  • estimation;
  • effect sizes;
  • confidence intervals;
  • meta-analysis;
  • precision