7.2 Odds ratios. Consider a 2×2 contingency table from a prospective study in which people who were or were not exposed to some pollutant are followed up and, after several years, categorized according to the presence or absence of a disease. Table 7.15 shows the probabilities for each cell. The odds of disease for either exposure group is Oi = πi/(1−πi), for i = 1,2, and so the odds ratio