Question: An epidemiologic study of breast cancer among elderly women followed 1,400 women, 65–79 years of age, without evidence of breast cancer over several years. Every two years the investigators examined the 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 The investigators also noted that 35 of the women had withdrawn from the study at the third evaluation. Based on this information, calculate (per person-year) and interpret the incidence density of breast cancer in this group.