Power, effect and sample size using GPower: practical issues for researchers and members of research ethics committees

JB Cunningham, Evie Gardner

    Research output: Contribution to journalArticle

    60 Citations (Scopus)


    Background. The issue of sample size has become a dominant concern for UK research ethics committees since theirreform in 2004. Sample size estimation is now a major, but often misunderstood concern for researchers, academicsupervisors and members of research ethics committees.Aim. To enable researchers and research ethics committee members with non-statistical backgrounds to use freelyavailable statistical software to explore and address issues relating to sample size, effect size and power.Method. Basic concepts are examined before utilising the statistical software package GPower to illustrate the use ofalpha level, beta level and effect size in sample size calculation. Examples involving t-tests, analysis of variance (ANOVA)and chi-square tests are used.Results. The examples illustrate the importance of effect and sample size in optimising the probability of a study to detecttreatment effects, without requiring these effects to be massive.Conclusions. Researchers and research ethics committee members need to be familiar with the technicalities of samplesize estimation in order to make informed judgements on sample size, power of tests and associated ethical issues. Alphaand power levels can be pre-specified, but effect size is more problematic. GPower may be used to replicate the examplesin this paper, which may be generalised to more complex study designs.
    Original languageEnglish
    Pages (from-to)132-136
    JournalEvidence Based Midwifery
    Publication statusPublished - 2007


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