Table 2

Relative risks for breast cancer among female military employees after night shift work by different metrics

Night shift work characteristic	Cases^* (N=141)		Controls^* (N=551)		OR^† (95% CI)	OR^‡ (95% CI)
Night shift work characteristic	n	%	n	%	OR^† (95% CI)	OR^‡ (95% CI)
Never	89	63	361	66	1 (Reference)	1 (Reference)
Ever	43	31	144	26	1.3 (0.8 to 2.0)	1.4 (0.9 to 2.1)
Duration (years^§)
Never	88	63	361	66	1 (Reference)	1 (Reference)
1–5.9	13	9	67	12	0.9 (0.5 to 1.7)	0.9 (0.4 to 1.7)
6–14.9	18	13	48	9	1.6 (0.9 to 3.0)	1.7 (0.9 to 3.2)
≥15	12	9	29	5	1.8 (0.9 to 3.7)	2.1 (1.0 to 4.5)
p For trend					0.06	0.03
Cumulative^¶
Never	82	58	351	64	1 (Reference)	1 (Reference)
<416	9	6	48	9	0.8 (0.4 to 1.8)	0.8 (0.4 to 1.9)
416–1560	14	10	48	9	1.4 (0.7 to 2.7)	1.4 (0.7 to 2.9)
>1560	17	12	43	8	1.9 (1.0 to 3.5)	2.3 (1.2 to 4.6)
p For trend					0.05	0.02
Duration and frequency (per week)
Never	82	58	351	64	1 (Reference)	1 (Reference)
1–2 times; all durations	15	11	65	12	1.0 (0.5 to 1.9)	1.0 (0.5 to 1.9)
≥3 times; 1–5.9 years	9	6	48	9	0.9 (0.4 to 2.0)	1.1 (0.5 to 2.3)
≥3 times; 6–14.9 years	11	8	30	5	1.8 (0.8 to 3.8)	2.1 (1.0 to 4.8)
≥3 times; ≥15 years	9	6	16	3	2.4 (1.0 to 5.6)	2.5 (1.0 to 6.6)
p For trend					0.06	0.02

↵* Numbers may not add up owing to missing values.
↵† Adjusted for age.
↵‡ Adjusted for age, hormone replacement therapy (0, 1–5, ≥6 years), number of childbirths (0, 1–2, ≥3), age at menarche (<11, 11–14, ≥14 years), years of education (7, 8–9, 10, ≥11), occasional sunbathing frequency (never/rare vs always/more than weekly) and tobacco smoking status (never, current, former).
↵§ Regardless of number per week.
↵¶ Based on tertiles for exposed controls.