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Table 1 Baseline characteristics of the participants by status in the UK and Norwegian studiesa

From: Adjusting for BMI in analyses of volumetric mammographic density and breast cancer risk

UK case control study Norwegian cohort study
  Controls (n = 685) Cases (n = 414) Non-cases
(n = 61,059)
Cases
(n = 657)
Age at mammography
 Mean (SD) 59.5 (6.6) 67.5 (12.7) 56.9 (5.74) 57.7 (5.43)
 Number 679 412 61,059 657
BMIb
 Mean (SD) 26.1 (5.6) 26.4 (4.9) 25.6 (4.2) 25.8 (4.1)
 Number 656 368 54,345 589
Ethnicity (UK)/Country of birth (Norway), n (%)
 White/Norway 520 (76.5) 370 (89.4) 56,234 (93.8) 612 (94.2)
 Non-white/Outside Norway 160 (23.5) 39 (9.6) 3693 (6.2) 38 (5.8)
 Missing 5 5 1132 7
Family history of BC, n (%)
 No N/A N/A 45,168 (77.1) 447 (70.0)
 Yes N/A N/A 13,390 (22.9) 192 (30.0)
 Missing    2501 18
Menopausal statusc, n (%)
 Pre- + peri-menopausal 91 (13.3) 55 (13.3) 14,776 (25.2) 141 (22.1)
 Post-menopausal 591 (86.7) 358 (86.7) 43,856 (74.8) 496 (77.9)
 Missing 3 1 2427 20
Parity, n (%)
 Nulliparous 209 (30.9) 65 (15.9) 4946 (8.5) 57 (9.0)
 Parous 467 (69.1) 343 (84.1) 53,563 (91.5) 577 (91.0)
 Missing 9 6 2550 23
Age at menarche in years, n (%)
  <13 271 (53.9) 159 (54.1) 16,764 (40.9) 186 (41.9)
 14+ 232 (46.1) 135 (45.9) 24,202 (59.1) 258 (58.1)
 Missing 14 33 4107 43
Hormone therapy use, n (%)
 No 459 (68.8) 246 (63.2) 34,150 (66.2) 305 (55.6)
 Yes 208 (31.2) 143 (36.8) 17,418 (33.8) 244 (44.4)
 Missing 18 25 9491 108
Educational level, n (%)
 None/primary school 35 (5.2) 17 (6.2)   
 Lower secondary    13,772 (23.3) 164 (25.9)
 Secondary or higher 641 (94.8) 225 (93.8) 45,457 (76.7) 470 (74.1)
 Missing 9 142 1830 23
Breastfeeding among parous women, n (%)
 Yes 358 (76.7) 224 (74.7) 46,107 (99.9) 497 (100)
 Missing 3 43 9929 103
  1. Abbreviations: BC breast cancer, BMI body mass index, N/A data not available, SD standard deviation
  2. aPercentages calculated without missing values
  3. bBMI estimated from self-reported height and weight as weight/height2 (in kg/m2)
  4. cPost-menopausal women defined as those who self-reported natural (cessation of menses for at least 12 months) or surgical menopause, were older than 55 years, or had ever used hormone therapy. Owing to small numbers, pre-menopausal (younger than 55 years and still having regular periods) and peri-menopausal (younger than 55 years and having irregular periods) women were combined into a single category