Statistics for clinicians. 2. Nominal data (I).

Nominal data consist of items assigned to well defined classes. They are presented as a proportion or percentage of the total. From a sample percentage we can estimate the population percentage with a desired degree of confidence, using standard error of the percentage as a measure of chance variation. We can compare the difference in percentage between two groups by means of the z-test or the chi-square test. If the number of observations is less than 40, Fisher's test is preferable to the chi-square test. When chi-square test is done on 2x2 tables, Yates' correction is recommended. For tables larger than a 2x2, Yates' correction is not used. When testing paired data for significance of difference, we need to use McNemar's modification of Chi-square test. Chi-square test is useful not only to test the significance of differences but also to test the significance of an association between attributes.