The researchers of the present study contacted the researchers who led the 20 past studies and collected information on whether each patient's infection was resistant to any of three antibiotics (clarithromycin, metronidazole, and amoxicillin). They also collected patients' characteristics, such as age, state of residence, sex, ethnicity, presence or past occurrence of a stomach ulcer, and method used to test for H. pylori. They then calculated the frequency of resistant H. pylori infection and searched for associations between the patient characteristics and resistance to antibiotics.