Statistics review 8: Qualitative data - tests of association

This review introduces methods for investigating relationships between two qualitative (categorical) variables. The chi(2) 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.