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Statistics review 8: Qualitative data – tests of association

Viv Bewick1*, Liz Cheek1 and Jonathan Ball2

Author Affiliations

1 Senior Lecturer, School of Computing, Mathematical and Information Sciences, University of Brighton, Brighton, UK

2 Lecturer in Intensive Care Medicine, St George's Hospital Medical School, London, UK

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Critical Care 2004, 8:46-53 doi:10.1186/cc2428

Published: 30 December 2003

Abstract

This review introduces methods for investigating relationships between two qualitative (categorical) variables. The χ2 test of association is described, together with the modifications needed for small samples. The test for trend, in which at least one of the variables is ordinal, is also outlined. Risk measurement is discussed. The calculation of confidence intervals for proportions and differences between proportions are described. Situations in which samples are matched are considered.

Keywords:
χ2 test of association; Fisher's exact test; McNemar's test; odds ratio; risk ratio; Yates' correction