Table 2.

Characteristics of the study population.

Characteristic*	Cases (N =450)	Controls (N =890)	P Value
Entry histology – no. (%)			< 0.0003
No proliferative disease	225 (50)	537 (60.3)
Proliferative disease	225 (50)	353 (39.7)
Age at entry biopsy – median (IQR†)	45 (38 – 50)	45 (38 – 50)	0.93
Year of entry biopsy – median (IQR†)	1969 (1963 – 1975)	1969 (1963 – 1975)	0.77
Years of follow-up – median (IQR†)	16 (9 – 25)	16 (9 – 25)	1
Family history of breast cancer – no. (%)			0.03
None	298 (68.8)	660 (75.5)
Second degree	51 (11.8)	85 (9.7)
First degree	84 (19.4)	129 (14.8)
Age at menarche – mean (SD‡)	12.9 (1.5)	13 (1.5)	0.168
Children			0.008
No	118 (26.2)	177 (19.9)
Yes	332 (73.8)	712 (80.1)
Age at first birth – median (IQR†)	24 (21 – 27.5)	23 (21 – 27)	0.22
Entry body mass index – median (IQR†)	21.9 (19.8 – 24.9)	22.3 (20.4 – 24.7)	0.47
Entry menopausal status – no. (%)			0.38
Premenopausal	199 (52.8)	383 (50)
Postmenopausal	178 (47.2)	383 (50)

^*There was no missing data for entry histology, age at entry biopsy, year of entry biopsy, or years of follow-up. The percent of missing observations in cases and controls, respectively for the other variables was as follows: family history 3.8%, 1.8%; age of menarche 15%, 9.2%; children 0%, 0.07%; age at first birth 27%, 21%; entry body mass index 47%, 43%; entry menopausal status 16%, 14%.

^†Inter-quartile range

^‡Standard deviation