Question

16. An epidemiologic study of breast cancer among elderly women followed 1,000 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 evaluation, 11 new cases at the second evaluation, 23 new cases at the third evaluation, and 28 new cases at the fourth (and final) evaluation. The investigators also noted that 40 women had withdrawn from the study at the third evaluation. Based on this information, calculate the person-time incidence rate (in person-years) of breast cancer in this group. Show all of your work.

Answer 1

The study results could also be described as follows: Breast cancer was diagnosed in 12 women at the first evaluation, in 11 women at the second evaluation, in 23 women at the third evaluation and 28 women in final evaluation. 40women withdrawed during third evaluation.

Calculate the incidence rate of breast cancer among this cohort. Assume that persons with new diagnoses of disease and those lost to follow-up were disease-free for one and half year, and thus contribute 1½ year to the denominator.

• Numerator = number of new cases of breast cancer

= 12 + 11 + 23 + 28

= 74

• Denominator = person-years of observation
  = 1000 + 1000 + 960 + 1000
  = 3960 person-years of follow-up
• Person-time rate =Number of new cases of disease or injury during specified period / Time each person was observed, totaled for all persons

= 74  ⁄ 3960
= 0.0186 cases per person-year
= 18.6 cases per 1,000 person-years