Acohort study of breast cancer among elderly women followed 1,400 women (65–79 years of age) with no evidence of breast cancer at baseline over several years. follow-up visits occurred every two years at which time, investigators screened all women for breast cancer. the results were as follows: 12 new cases at the first follow-up visit, 11 new cases at the second follow-up visit, 23 new cases at the third follow-up visit, and 28 new cases at the fourth (final) follow-up visit. the investigators also noted that 35 of the women had withdrawn from the study after the third evaluation (these women were present for visits 1 - 3 but missed the 4th visit). based on this information, calculate and interpret the incidence density (per 1,000 person-years) of breast cancer in this group.