with functional abnormalities only in some cases resolve without becoming clinically manifest (e.g., aortic valve with two leaflets, the prolapsing mitral valve, silent persistent patency of the arterial duct or small septal defects, included patency of the oval foramen) Ratios, Rates, and Proportions Confusion and misuse regarding the terms incidence and prevalence are rooted in misunderstanding regarding what constitutes a ratio, a rate, and a proportion These definitions are summarized in Table 13.2 Table 13.2 Statistical and Methodologic Definitions, Some Considerations, and Examples Used in Identifying the Frequency of Congenital Cardiac Disease Term Ratio Definition, Some Considerations, and Examples A relationship between two quantities where the numerator and a denominator are mutually exclusive (e.g., the ratio of males to females with transposed arterial trunks) Proportion The frequency of a condition or category within the population specified in the denominator independent of time (e.g., the proportion of muscular septal defects among all ventricular septal defects) Rate The frequency of occurrence of a condition that takes into account the time period over which the numerator was accumulated (e.g., the rate of the occurrence of a thrombotic episode in patients during their first year subsequent to the Fontan operation) Kaplan-Meier The cumulative frequency of occurrence of a condition that reflects instantaneous changes in rates estimates and risk over time and, unlike rate, does not assume that the condition occurs uniformly over the specified period of time (e.g., freedom from of a thrombotic episode in patients subsequent to the Fontan operation over 10-year period, showing maximal occurrence in first postoperative year) Incidence The rate of new occurrences of a condition over a specified period of time in a population at risk (e.g., new cases per year of Kawasaki disease per 100,000 children