Fig. 18.2, taken from Danesh et al. (1999), is a summary of studies of H. pylori seropositivity in relation to the occurrence of coronary heart disease. (Three other retrospective studies are not included since they did not report separate results for cases of myocardial infarction. Together, these studies included fewer than 150 cases of myocardial infarction.) Black squares indicate odds ratio, with area of square proportional to number of cases, and horizontal lines represent confidence intervals. From this figure, assume that the most trustworthy data bearing on a possible association between H. pylori seropositivity and the occurrence of coronary heart disease come from:
• The ISIS study for persons 30-49 years; and
• The four prospective studies for persons older than this.
Also assume that the studies were done in a comparable way in the same geographic population; that H. pylori seropositivity is a faithful indication of H. pylori infection, regardless of age; and that the difference in the two odds ratios (2 vs. 1.2) is not due to chance. These results do not necessarily indicate that 30–49-year-old persons are more susceptible to the harmful influence of H. pylori infection on the occurrence of coronary heart disease than are persons beyond this age. Why?
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