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Data Analysis. Follow-up time was calculated as the number of months between joining the cohort (i.e., the date the baseline questionnaire was completed) and either the date of invasive breast cancer diagnosis, the diagnosis of in situ breast cancer, the date of death, the date (or estimated date) the woman moved out of California, or December 31, 2000, whichever came first. In situ breast cancer cases were censored (rather than included in the analysis with invasive cases) for comparability with previous studies, because the type of treatment for in situ breast cancer may affect the risk of subsequent invasive breast cancer to varying degrees, and because there may be differences in the etiological factors for these two stages of breast cancer. In this cohort, in situ diagnoses are largely ductal carcinoma in situ (DCIS) which are detected primarily by mammogram, the biological significance of which is not clear. Relative risks (RRs; hazard rate ratios) and 95% confidence intervals (CIs) were estimated using Cox proportional hazards regression models with time-on-study used as the timescale and with age adjusted for linearly within each of two age groups, that is, thus allowing for different slopes for younger (<50 years) and older ( 50 years) women. This approach produced point estimates similar to those observed with the more computationally intense approach of using age as the timescale (i.e., where subjects enter the risk set at the age they filled out the baseline questionnaire and exit at their event/censoring age) with adjustment for birth cohort (calendar time) effects. RRs for alcohol consumption of 20 g/day versus nondrinkers were of primary interest based on previous findings (4). RRs were adjusted for race/ethnicity (white, nonwhite), daily caloric intake (linearly), a family history of breast cancer in a first degree relative (yes/no), age at menarche (<12, 12), nulliparity/age at first full-term pregnancy (FFTP) (nul-liparous, FFTP V24, 25 – 29, 30), physical activity (hours per week modeled linearly), body mass index (BMI; <25.8, 25.8 – 32.2, 32.3 kg/m2), and use and duration of estrogen replacement therapy (ERT; none, V5 years, >5 years) (12, 13). Note: Hormone replacement therapy (HRT) analyses in Tables 3 and 4 are based on combination HRT; those reported in the text for the duration and timing (current versus former) of use are based on ERT use only because this information was not available for progestin use. Effect modification under a multiplicative model was formally assessed using meth-ods described by Walter and Holford (14). Two-sided P values for assessing interaction were based on the term comparing nondrinkers to heavy drinkers ( 20 g/day).
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