Table 2 Standardised incidence ratios and 95% CI of Crohn's disease among 709 353 patients undergoing appendectomy in Swedish (1964 to 2004) and Danish (1977 to 2004) hospitals.

	Person‐years	Obs	Exp	SIR (95% CI)	p Value*
Overall	11 078 855	1655	1089.7	1.52 (1.45 to 1.59)	to
Country					0.054
Sweden	7 250 881	1070	729.0	1.47 (1.38 to 1.56)
Denmark	3 827 974	585	360.7	1.62 (1.50 to 1.76)
Sex					0.42
Women	6 467 929	1052	682.3	1.54 (1.45 to 1.64)
Men	4 610 926	603	407.4	1.48 (1.37 to 1.60)
Age at appendectomy (years)
0 to 9	952 028	77	77.1	1.00 (0.80 to 1.25)	<0.001
10 to 19	3 358 272	533	370.1	1.44 (1.32 to 1.57)
20 to 29	2 376 137	452	267.2	1.69 (1.54 to 1.85)
30 to 39	1 672 986	236	150.7	1.57 (1.38 to 1.78)
40 to 49	1 267 122	163	105.1	1.55 (1.33 to 1.81)
⩾50	1 452 309	194	119.5	1.62 (1.41 to 1.87)
Age at risk of Crohn's disease (years)
0 to 9	144 077	7	0.9	7.95 (3.79 to 16.68)	<0.001
10 to 19	1 357 510	204	71.9	2.88 (2.51 to 3.30)
20 to 29	2 425 751	538	370.9	1.45 (1.33 to 1.58)
30 to 39	2 344 822	329	245.6	1.34 (1.20 to 1.49)
40 to 49	1 786 427	224	147.9	1.51 (1.33 to 1.73)
⩾50	3 020 268	353	253.5	1.39 (1.25 to 1.55)

Exp, Expected number of patients with Crohn's disease; Obs, observed number of patients with Crohn's disease; SIR, standardised incidence ratio.

*Test of homogeneity.