Table 2.
Selected characteristics of restricted cohorta by race.
| Variable | White (n = 2,859) | Nonwhite (n = 282) |
|---|---|---|
| Cases and noncases | ||
| Year of birth (mean ± SD) | 1934 ± 11.6 | 1933 ± 10.6 |
| Self-respondent on questionnaire (%) | 90.0 | 89.0 |
| Ever-smoker (%) | 51.8 | 55.0 |
| Pack-years (ever-smokers only) (mean ± SD) | 35.2 ± 28.0 | 29.3 ± 24.0* |
| BMI at 20 years of age [kg/m2 (mean ± SD)] | 21.2 ± 3.2 | 21.5 ± 3.6 |
| Age at menarche [years (mean ± SD)] | 12.9 ± 1.7 | 13.2 ± 1.9 |
| Premenopausal at diagnosis (%) | 19.2 | 14.3 |
| Parous (%) | 87.5 | 88.0 |
| No. of live births (mean ± SD) | 2.5 ± 1.7 | 2.9 ± 2.1** |
| Age at first live birth [years (mean ± SD)] | 23.0 ± 4.4 | 22.5 ± 4.7 |
| Used hormone replacement therapy (%) | 33.2 | 31.6 |
| Age began hormone replacement use [years (mean ± SD)] | 47.3 ± 8.9 | 48.2 ± 10.4 |
| Breast cancer in first-degree female relative (%) | 13.5 | 18.8* |
| Years exposed (mean ± SD) | 7.0 ± 7.1 | 6.6 ± 6.6 |
| Cumulative exposure (1,000 unit-years) | ||
| Mean ± SD | 0.45 ± 0.81 | 0.64 ± 0.88** |
| Median | 0.16 | 0.33** |
| Breast cancer cases only | ||
| No. of cases | 131 | 14 |
| Cumulative exposure (1,000 unit-years) | ||
| 25th percentile | 0.07 | 0.17 |
| 50th percentile | 0.22 | 1.15 |
| 75th percentile | 0.63 | 2.13 |
a
Restricted cohort comprises women with questionnaire data for ever smoking, parity, age at first live birth, breast cancer in a first-degree female relative, hormone use, and age began hormone use. Some other variables have missing values.
*
p < 0.05
**
p < 0.01.