Table 2. Crude and covariate-adjusted hazard ratios (HRs) for gallstones among sampled subjects during the 5-year follow-up period.

Presence of gallstones	Total sample (N = 9832)	Subjects who underwent an appendectomy (N = 4916)	Comparison subjects (N = 4916)
Five-year follow-up period
Yes, n (%)	163 (1.66)	105 (2.14)	58 (1.18)
Incidence rate per 1000 person-years (95% CI)	3.65 (3.11~4.25)	4.71 (3.85~5.70)	2.59 (1.97~3.35)
Crude HR (95% CI)	-	1.83*** (1.32~2.53)	1.00
Adjusted a HR (95% CI)	-	1.79*** (1.29~2.48)	1.00

Notes: CI, confidence interval. The HR was calculated by a stratified Cox proportional hazard regression which was stratified by sex, age group, and the year of the index date.

^a Adjustments were made for subjects’ monthly income, geographic region, hypertension, coronary heart disease, hyperlipidemia, and diabetes

*** p<0.001.