In a study of adverse effects of radiotherapy among children in the Gambia, 10.834 irradiated children were identified from original treatment records and matched to 10,834 non-irradiated comparison subjects selected from the general population. Subjects were followed for a mean of 26 years.

Person-years of observations: irradiated subjects,279,901 person-years; comparison subjects 280,561 person-years. During the follow-up period there were 49 deaths from cancer in irradiated subjects, and 44in the non-irradiated population comparison subjects.

5. What type of study design is this?

6.what are the rates of cancer death (per 100,000 person-years) in each of the 2 groups?

7. Calculate and interpret the appropriate relative measure of association estimating the relationship between cancer death and irradiation exposure.

8. Assuming causality, how many cancer deaths per 100.000 irradiated subjects per year were due to the effect of radiotherapy?

9.What is this measure referred to above called?

10. Again assuming causality, what proportion of cancer deaths in irradiated subjects can be attributed to the effect of radiation?