Figure 2.

The Vioxx risk pattern (top row) occurs when a patient with rheumatoid arthritis (RA) who takes Vioxx and suffers a myocardial infarction (MI)—these frequencies are entered into cell a of the 2x2 contingency matrix. Based on the various combinations of temporal orderings for the initial mentions of RA, Vioxx, and MI in the patient notes, other possible patterns contribute to the expected background distribution (cells b, c, d of the contingency table) that the odds ratio calculation requires.