Study group populations for the PF dataset.

There are 227,438 patients with gastroesophageal reflux disease (GERD) who are at least 18 years old in this confirmation cohort. In all study groups other than the one in which clopidogrel patients are excluded, the populations are matched based on clopidogrel use and other covariates such as age, gender, race, length of observation, and number of unique drugs mentioned in the record as well as the number of unique disease concepts (as proxies for health status). The matching process attempts to achieve a ratio of 1:5 (exposure to control), but will settle for 1:3, 1:2, or 1:1 if there are not enough controls from which to draw. The length of observation in the PF dataset makes balancing variables difficult, so confounding cannot be entirely ruled out even after matching.† patients on clopidogrel are excluded‡ patients are restricted by ageStudy group populations for the PF dataset.