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When clinicians consider the results of clinical trials, they are interested in the association between a treatment and an outcome. This chapter will help you understand and interpret study results related to outcomes that are either present or absent (dichotomous or binary) for each patient. Such binary outcomes include death, stroke, myocardial infarction, hospitalization, or disease exacerbations. A guide for teaching the concepts in this chapter is also available.1

The 2 × 2 Table

Table 9-1 is a 2 × 2 table that captures the information for a dichotomous outcome of a clinical trial.


The 2 × 2 Table

For instance, during a randomized trial that compares mortality rates in patients with bleeding esophageal varices that were controlled by endoscopic ligation or endoscopic sclerotherapy,2 18 of 64 participants assigned to ligation died, as did 29 of 65 patients assigned to sclerotherapy (Table 9-2).


Results From a Randomized Trial of Endoscopic Sclerotherapy Compared With Endoscopic Ligation for Bleeding Esophageal Varicesa

The Risk

The simplest measure of occurrence to understand is the risk (or absolute risk). We often refer to the risk of the adverse outcome in the control group as the baseline risk, the control group risk, or, occasionally, the control event rate.

The risk of dying in the ligation group is 28% (18/64 or [a/(a + b)]), and the risk of dying in the sclerotherapy group is 45% (29/65 or [c/(c + d)]).

The Risk Difference (Absolute Risk Reduction)

One way of comparing 2 risks is by calculating ...

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