Statistics for clinicians. 3. Nominal data (II)

ABSTRACT

When the percentage occurrence of an event in a series of groups shows a linear trend, the standard chi-square test may not reveal its significance. The data should then be analyzed by z-test for a linear trend. When the results of several similar experiments show a trend in favor of one group, but the differences in individual studies are not significant, the data from all studies can be analyzed together by the z-test for significance of the trend. In some studies, patients need to be followed up for long periods for the occurrence of an event. Here, we are interested not only in the frequency of the event, but also how soon it occurs or how long it is delayed. For this purpose the data should be analyzed by the life table method (also known as the logrank method or the Mantel-Haenszel method). Diagnostic tests are usually evaluated by their sensitivity and specificity. However, what is also important for a clinician is their predictive value, which depends on the prevalence of the condition.