Table 1.

Distribution of Person-Years Contributed by Sister Study Participants According to Breast Cancer Diagnosis and Select Characteristics, Sister Study

	Cases (N) N=2,134	Person-Years (PY) PY=270,886	Distribution of Person Years (% PY)
Birth cohort			Cases	Non-cases
1928–1946	778	79,389	35.9	29.1
1946–1955	809	107,160	39.1	39.6
1956–1965	491	70,715	22.4	26.2
1966–1974	56	13,622	2.6	5.1
BMI
<18.5	19	3,114	0.8	1.2
18.5–24.9	761	103,339	36.2	38.2
25.0–29.9	685	85,650	32.6	31.6
≥30.0	669	78,688	30.4	29.0
Race
Non-Hispanic White	1,850	232,444	88.5	85.7
Non-Hispanic Black	153	20,636	6.1	7.7
Hispanic	66	10,810	2.2	4.0
Other/unknown	64	6,929	3.2	2.5
Parity/breastfeeding
Nulliparous	397	4,9752	18.5	18.4
Parous, never breastfed	547	6,6029	25.2	24.4
Parous, breastfed	1,190	154,920	56.3	57.2
Unknown	0	185	0.0	0.1
Menopausal hormone therapy
Never	1,153	154,130	55.0	56.9
Estrogen (E) only	409	53,668	18.2	19.9
Estrogen and Progestin (E+P) only	462	51,538	21.6	19.0
E and E+P	106	10,816	5.0	4.0
Unknown	4	733	0.2	0.3