Table 1.

Selected baseline characteristics of the study population

VariableaCases (N = 497)Subcohort (N = 2,830)P
Age, y65 (59–70)63 (57–69)<0.0001
WHI, N (%)
 Observational study cohort401 (80.7)756 (26.7)N/A
 Clinical trial cohort96 (19.3)2,074 (73.3)
Ethnicity, N (%)
 White394 (79.3)1,591 (56.4)<0.0001
 Black60 (12.1)634 (22.4)
 Hispanic22 (4.4)340 (12.0)
 Asian/other19 (3.8)263 (9.2)
 Missing2 (0.4)2 (0.1)
BMI (kg/m2), N (%)0.17
 Normal (BMI < 25.0)155 (31.2)762 (26.9)
 Overweight (25.0 ≤ 30.0)165 (33.2)998 (35.3)
 Obese (≥30.0)177 (35.6)1,070 (37.8)
Age at menarche, N (%)0.44
 ≤1041 (8.2)193 (6.8)
 11–12203 (40.8)1,101 (38.9)
 13+250 (50.3)1,523 (53.8)
Missing3 (0.7)13 (0.5)
Age at menopause, N (%)0.001
 ≤4272 (14.5)541 (19.1)
 43–48106 (21.3)629 (22.2)
 49–51133 (26.8)616 (21.8)
 ≥52142 (28.6)675 (23.9)
 Missing44 (8.8)369 (13.0)
Parity, N (%)0.04
 079 (15.9)334 (11.8)
 139 (7.8)254 (9.0)
 ≥2375 (75.5)2,223 (78.6)
 Missing4 (0.8)19 (0.6)
Family history of breast cancer, N (%)<0.0001
 Yes138 (27.8)440 (15.5)
 No185 (37.2)2,164 (76.5)
 Missing174 (35.0)226 (8.0)
Smoking status, N (%)0.001
 Never254 (51.1)1,519 (53.7)
 Former213 (42.9)1,021 (36.1)
 Current22 (4.4)250 (8.8)
 Missing8 (1.6)40 (1.4)
Education history, N (%)<0.0001
 High school and less144 (29.0)1,125 (39.8)
 College203 (40.8)982 (34.7)
 Postgraduate education143 (28.8)701 (24.8)
 Missing7 (1.4)22 (0.7)
Alcohol (servings/week)0.4 (0.0–3.5)0.2 (0.0–1.4)<0.0001
Physical activity (METsb)8.29 (2.00–17.50)5.75 (0.50–15.00)<0.0001

NOTE: P values derived from the Wilcoxon rank-sum test for continuous data and Pearson χ2 for categorical data.

  • aValues are medians (interquartile range) unless otherwise stated.

  • bMET, metabolic equivalent tasks (defined as the caloric need per kilogram of body weight per hour of activity divided by the caloric need per kilogram of body weight per hour at rest) per hour per week.