Table 1.

Distribution of baseline demographic characteristics, breast cancer case groups and non-cases

	Non-cases	Cases*
		ER+	Triple-negative
	n (%)	n (%)	n (%)
Age at randomization / enrollment
50–59	50,445 (33)	746 (29)	122 (40)
60–69	67,606 (45)	1,250 (48)	125 (41)
70–79	32,654 (22)	614 (24)	60 (20)
Race / ethnicity
White, not of Hispanic origin	124,164 (83)	2,317 (89)	241 (79)
Hispanic/ Latina	6,089 (4)	53 (2)	8 (3)
African-American	13,681 (9)	149 (6)	50 (16)
Other	6,391 (4)	85 (3)	7 (2)
Missing	380	6	1
Education
≤High school diploma / GED	33,967 (23)	483 (19)	75 (25)
Vocational / training school	15,370 (10)	232 (9)	30 (10)
Some college / associates degree	41,558 (28)	696 (27)	78 (26)
≥College graduate	58,684 (39)	1,177 (45)	118 (39)
Missing	1,126	22	6
Household income level
<$35,000	57,879 (41)	910 (37)	108 (38)
$35,000 – $74,999	56,720 (40)	1,023 (42)	126 (44)
≥$75,000	26,004 (18)	507 (21)	50 (18)
Missing	10,102	170	23
History of breast cancer in female relatives
No	116,762 (82)	1,901 (77)	207 (72)
Yes	25,713 (18)	574 (23)	81 (28)
Missing	8,230	135	19
Hormone therapy (HT) use at baseline
No	64,636 (43)	975 (37)	138 (45)
Yes	85,775 (57)	1,633 (63)	169 (55)
Missing	118	2	0
History of mammography in past 2 years
No	24,242 (17)	327 (13)	43 (15)
Yes	121,520 (83)	2,212 (87)	253 (85)
Missing	4,767	71	11
Parity
Nulliparous	17,509 (12)	380 (15)	23 (8)
1	13,151 (9)	248 (10)	20 (7)
2	37,355 (25)	683 (26)	90 (29)
≥3	81,558 (55)	1,279 (49)	173 (57)
Missing	956	20	1
Age at first birth (parous women only)
<20	19,365 (16)	258 (13)	43 (18)
20–29	87,961 (74)	1,481 (74)	184 (76)
≥30	10,861 (9)	255 (13)	16 (7)
Missing	13,877	216	40

^*ER+ = estrogen receptor-positive cases with known HER2 expression status; triple-negative = ER−/PR−/HER2−