XVI. The process of prognostic stratification (Part 2)

A partition consists of the array of categories that express the values of a variate. The categories can be demarcated with verbal specifications (such as men), with numerical boundaries (such as above age 65), or with both (men above age 65). The groups of people delineated by the categories of a variate are called strata. When the rates of the subsequent target event in a cohort are associated with the initial strata, the variate is called a predictor variate, and the result is called a prognostic stratification. Since the data of clinical epidemiology are expressed in many individual variates, and since stratifications can be formed for each of the observed variates as well as for diverse multivariate combinations, we needed to establish methods for determining the effectiveness of a partition and for deciding when and how to alter it. A partition can be changed in several ways. If the number of members in certain strata is below a modicum size (such as 20), the strata can be either eliminated by rearrangement of the partition's demarcation boundaries, or enlarged by combination with other strata. The strata chosen for combination should preferably have two types of homogeneity: They should be isometric in the similarity of their target rates, and concordant in the similarity of their biologic characteristics. De-