Figure 1.

Design of the study. Four reference subjects were retrieved for each index subject with an appendicectomy after the onset of ulcerative colitis. Reference subjects are ulcerative colitis patients, matched with respect to sex, age (within five years), and time of onset of ulcerative colitis (within two years) but who did not have an appendicectomy. They were assigned an index date equidistant to the appendicectomy date of the index subject and followed up for a variable amount of time thereafter.