Table I.
Characteristics of case and control groups.
| Variables | Control (%) | Case (%) | χ2 | P-value |
|---|---|---|---|---|
| Residence | 47.14 | <0.001 | ||
| Rural | 510 (37.14) | 617 (50.53) | ||
| Urban | 863 (62.86) | 604 (49.47) | ||
| Age, years | 4.58 | 0.101 | ||
| 25–44 | 588 (40.25) | 467 (36.31) | ||
| 45–59 | 751 (51.40) | 700 (54.43) | ||
| 60–70 | 122 (8.35) | 119 (9.25) | ||
| Education | 42.98 | <0.001 | ||
| Junior high school or lower | 578 (40.93) | 669 (53.65) | ||
| Senior high school or above | 834 (59.07) | 578 (46.35) | ||
| Monthly family income, RMB | 28.03 | <0.001 | ||
| <5,000 | 946 (66.71) | 954 (76.02) | ||
| ≥5,000 | 472 (33.29) | 301 (23.98) | ||
| BMI | 1.59 | 0.207 | ||
| <24 | 687 (50.29) | 576 (47.80) | ||
| ≥24 | 679 (49.71) | 629 (52.20) | ||
| Age of menarche, years | 9.40 | 0.024 | ||
| ≤12 | 116 (8.06) | 103 (8.17) | ||
| 13–14 | 669 (46.49) | 571 (45.32) | ||
| 15–16 | 459 (31.90) | 364 (28.89) | ||
| ≥17 | 195 (13.55) | 222 (17.62) | ||
| Number of births | 56.73 | <0.001 | ||
| 0 | 40 (2.78) | 27 (2.13) | ||
| 1 | 871 (60.44) | 613 (48.38) | ||
| 2 | 419 (29.08) | 439 (34.65) | ||
| ≥3 | 111 (7.70) | 188 (14.84) | ||
| Age at first childbirth, yearsa | 4.13 | 0.389 | ||
| <20 | 31 (2.27) | 40 (3.31) | ||
| 20–24 | 584 (42.75) | 530 (43.91) | ||
| 25–29 | 679 (49.71) | 577 (47.80) | ||
| 30–35 | 61 (4.47) | 54 (4.47) | ||
| ≥35 | 11 (0.81) | 6 (0.50) | ||
| Breast-feeding | 0.67 | 0.412 | ||
| No | 111 (7.94) | 109 (8.83) | ||
| Yes | 1,287 (92.06) | 1,126 (91.17) | ||
| Menopausal status | 8.61 | 0.003 | ||
| Premenopausal | 988 (70.42) | 813 (65.09) | ||
| Postmenopausal | 415 (29.58) | 436 (34.91) | ||
| History of breast cancer in first-degree relatives | 3.25 | 0.072 | ||
| No | 1,368 (97.30) | 1,190 (96.05) | ||
| Yes | 38 (2.70) | 49 (3.95) | ||
| Cigarette smoking | 5.91 | 0.052 | ||
| Never | 1,427 (97.87) | 1,238 (96.42) | ||
| Sometimes | 17 (1.17) | 21 (1.64) | ||
| Often | 14 (0.96) | 25 (1.95) | ||
| Alcohol drinking | 2.57 | 0.277 | ||
| Never | 1,278 (87.59) | 1,096 (85.65) | ||
| Sometimes | 169 (11.58) | 171 (13.35) | ||
| Often | 12 (0.82) | 14 (1.09) | ||
| Physical activity | 5.59 | 0.061 | ||
| Never | 410 (28.20) | 364 (28.44) | ||
| Sometimes | 663 (45.60) | 534 (41.72) | ||
| Often | 381 (26.20) | 382 (29.84) | ||
| Hormone receptor status | – | – | ||
| ER+ | – | 977 (75.97) | ||
| ER− | – | 309 (24.03) | ||
| PR+ | – | 888 (69.05) | ||
| PR− | – | 398 (30.95) | ||
| ER+PR+ | – | 868 (67.50) | ||
| ER−PR− | – | 289 (22.47) |
a
limited to participants who had children. BMI, body mass index; ER, estrogen receptor; PR, progesterone receptor.