Table 2

Association of acute appendicitis with ulcerative colitis in the 1968–1998 birth cohort (N=3 161 080)

Comparison	Model	HR (95% CI)	P value
Between-individual	Unadjusted*	0.44 (0.34 to 0.57)	<0.001
Between-individual	Adjusted†	0.37 (0.29 to 0.48)	<0.001
Within-sibling	Unadjusted*	0.47 (0.33 to 0.67)	<0.001
Within-sibling	Adjusted†	0.46 (0.32 to 0.66)	<0.001

*Unadjusted model
†Model adjusted for birth year, sex, county, HCI and SEI.
HCI, Household Crowding Index ; SEI, Socio-Economic Index .