Table 1. Distributions of select variables in breast cancer cases and cancer-free controls.

Variables	Stage 1		Stage 2		Combined		Combined
	case(N = 878)	control(N = 900)	case(N = 914)	control(N = 967)	case(N = 1792)	control(N = 1867)	P value
Age, year (mean ± SD)	51.29±11.38	51.47±11.67	50.11±11.36	48.64±12.28	50.69±11.38	50.01±12.07	0.077
Age at menarche, year (mean ± SD)a	15.25±1.85	15.85±1.90	15.18±1.96	16.24±1.81	15.21±1.90	16.05±1.86	<0.0001
Age at first live birth, year (mean ± SD)b	25.65±3.48	24.90±3.40	25.55±3.16	24.15±2.58	25.60±3.32	24.51±3.03	<0.0001
Age at natural menopause, year (mean ± SD)c	49.85±3.18	49.15±4.09	49.71±3.72	49.69±3.56	49.78±3.45	49.42±3.84	0.051
Menopausal status							<0.0001
Premenopausal	416	437	434	530	850	967
Natural menopause	386	432	379	418	765	850
Unnatural menopause	58	16	84	9	142	25
Estrogen receptor (ER)
Positive	369		434		803
Negative	321		322		643
Progesterone receptor (PR)
Positive	396		414		810
Negative	294		340		634

^aAge at menarche information was available in 862 breast cancer cases and 896 controls (stage 1), 893 breast cancer cases and 962 controls (stage 2).

^bAge at first live birth information was available in 830 breast cancer cases and 872 controls (stage 1), 850 breast cancer cases and 943 controls (stage 2).

^cAge at natural menopause information was available in 380 breast cancer cases and 411 controls (stage 1), 363 breast cancer cases and 398 controls (stage 2).