Table 1. Characteristics of breast cancer cases (overall and by ER status) and controls.

	Controls (N=84 778)	All cases (N=84 778)	ER-positive casesa (N=56 296)	ER-negative casesa (N=9991)
	%
Age in years (median)	(76.0)	(76.0)	(76.0)	(75.0)
67–70	19.2	19.1	19.2	21.3
71–75	28.0	28.3	28.6	29.4
76–80	24.9	25.3	25.8	24.4
81–84	14.6	13.9	16.4	15.2
85+	13.3	13.3	10.1	9.7
Race
White	83.7	88.5	89.8	84.7
Black	7.7	6.4	5.2	9.4
Hispanic	2.4	1.3	1.2	1.5
Others/unknown	6.2	3.8	3.8	4.4
Region of residence b
Western	51.2	51.3	52.8	47.9
Northeastern	17.0	17.6	16.6	18.8
North-central	20.1	20.1	20.0	21.1
Southern	11.7	11.0	10.6	12.2
Income c
<$25 000	20.8	18.7	18.3	20.2
$25 000–$34 999	28.1	28.1	28.5	28.2
$35 000–$49 999	32.4	33.0	33.0	32.4
⩾$50 000	13.2	14.0	14.2	13.4
Unknown	5.4	6.2	6.0	5.8
Selection year
1993–1995	22.5	22.5	21.1	23.3
1996–1999	31.5	31.5	31.9	32.5
2000–2002	46.0	46.0	47.0	44.2
Mammography d
Yes	45.4	50.5	52.0	48.7
No	54.6	49.5	48.0	51.3
Months of coveragee (median)	(102.0)	(101.0)	(102.0)	(98.0)
12–23	3.2	3.2	3.3	3.1
24–59	19.2	19.2	19.3	21.1
60+	77.6	77.6	77.4	75.8
Number of physician visitsf (median)	(11.0)	(12.0)	(12.0)	(12.0)
0	16.0	15.3	15.2	15.2
1–11	36.7	35.2	35.3	36.8
12–23	23.2	24.4	24.7	23.4
24+	24.1	25.1	24.8	24.6

^aER status is missing for 18 491 breast cancer cases

^bWestern region includes Hawaii, New Mexico, San Francisco, Seattle, Utah, San Jose, Los Angeles, and Greater California; North eastern region includes Connecticut and New Jersey; North-central includes Detroit and Iowa; Southern region includes Atlanta, rural Georgia, Kentucky, and Louisiana.

^cMedian household income by residential zip code.

^dEver had mammography 12–48 months before case–control selection.

^eNumber of months of total simultaneous coverage part A, B, and non-HMO.

^fNumber of visits to a physician 12–24 months before case–control selection.