Table 1. Distribution of cases and controls by potential risk factors, and results of univariate conditional logistic regression.
| Factor | Category | Cases (%) | Controls (%) | OR (95% CI) | Significance |
|---|---|---|---|---|---|
| Age (years) | <55 | 160 (33) | 320 (33) | NAa | NA |
| 55–59 | 196 (40) | 392 (40) | NA | ||
| 60+ | 135 (27) | 270 (27) | NA | ||
| Ethnicity | Chinese | 422 (86) | 844 (86) | NA | NA |
| Other | 69 (14) | 138 (14) | NA | ||
| Education | None | 239 (49) | 577 (59) | 1.00 (−) | P=0.001 |
| Primary | 93 (19) | 176 (18) | 1.29 (0.95–1.73) | ||
| Secondary | 107 (22) | 143 (15) | 1.83 (1.35–2.47) | ||
| A-level | 37 (7) | 54 (5) | 1.68 (1.06–2.64) | ||
| Higher | 15 (3) | 32 (3) | 1.24 (0.65–2.36) | ||
| Percent breast density | 0–10 | 33 (7) | 117 (12) | 1.00 (−) | P<0.001 |
| 11–25 | 76 (15) | 282 (29) | 1.04 (0.64–1.69) | ||
| 26–50 | 215 (44) | 391 (40) | 2.16 (1.39–3.35) | ||
| 51–75 | 151 (31) | 171 (17) | 3.59 (2.23–5.78) | ||
| 76–100 | 16 (3) | 21 (2) | 3.34 (1/54–7.20) | ||
| Age at menarche | ⩽14 | 295 (60) | 514 (52) | 1.00 (−) | P=0.003 |
| >14 | 196 (40) | 468 (48) | 0.71 (0.56–0.90) | ||
| Age at menopause | <50 | 149 (30) | 370 (38) | 1.00 (−) | P=0.008 |
| ⩾50 | 271 (55) | 503 (51) | 1.32 (1.03–1.68) | ||
| Premenopausal | 71 (15) | 109 (11) | 1.76 (1.17–2.63) | ||
| Parity | 0 | 70 (14) | 95 (10) | 1.00 (−) | P=0.005 |
| 1 | 30 (6) | 51 (5) | 0.78 (0.45–1.36) | ||
| 2 | 101 (21) | 167 (17) | 0.82 (0.55–1.23) | ||
| 3+ | 290 (59) | 669 (68) | 0.59 (0.41–0.83) | ||
| Age at first birth (parous only) | <20 | 62 (15) | 155 (17) | 1.00 (−) | P=0.01 |
| 20–24 | 146 (34) | 376 (42) | 0.46 (0.23–0.93) | ||
| 25–29 | 139 (33) | 248 (28) | 0.42 (0.22–0.81) | ||
| 30–34 | 54 (13) | 85 (10) | 0.64 (0.33–1.23) | ||
| 35+ | 20 (5) | 23 (3) | 0.62 (0.30–1.27) | ||
| History of breastfeeding | No | 164 (39) | 279 (31) | 1.00 (−) | P=0.006 |
| Yes | 257 (61) | 608 (69) | 0.69 (0.52–0.91) | ||
| Oral contraceptive use | No | 325 (66) | 602 (61) | 1.00 (−) | P=0.07 |
| Yes | 166 (34) | 380 (39) | 0.81 (0.64–1.02) | ||
| HRT (ever) | No | 401 (82) | 852 (87) | 1.00 (−) | P=0.008 |
| Yes | 90 (18) | 130 (13) | 1.52 (1.11–2.05) | ||
| Family history of breast cancer | No | 456 (93) | 956 (97) | 1.00 (−) | P<0.001 |
| Yes | 35 (7) | 26 (3) | 2.88 (1.69–4.89) | ||
| Body mass indexb | <21.5 | 92 (19) | 253 (25) | 1.00 (−) | P=0.03 |
| 21.5–23.89 | 114 (23) | 244 (25) | 1.21 (0.88–1.67) | ||
| 23.9–26.69 | 143 (29) | 248 (25) | 1.51 (1.10–2.08) | ||
| ⩾26.7 | 141 (29) | 245 (25) | 1.51 (1.10–2.07) |
Abbreviations: CI=confidence interval; HRT=hormone replacement therapy; NA=not applicable; OR=odds ratio.
a
Matching variable so inference not applicable.
b
By quartiles of the control group.