Follow-up for incident events was calculated as the age at completion of the 1992
questionnaire until the age at breast cancer diagnosis, age at move from Iowa, or age at
death. If none of these events occurred, a woman was assumed to be alive, cancer-free, and
living in Iowa through December 31, 2005. Cox proportional hazards regression analysis
was used to estimate relative risks (RRs) and 95% confidence intervals (CIs) for the
association of breast cancer risk with aspirin and NSAID use. Incidence was modeled as a
function of age [27].