In a study of the adverse effects of x-rays among children, a retrospective cohort study was done using records from several large children's hospitals for the period of 1980 to 1985. 10,000 children were selected as a representative population of ill children seen at the hospitals during that time. Subjects were classified according to whether or not they received an x-ray during their stay in the hospital and were followed from their hospital stay through 2005 for the development of cancer. During the follow-up period, 49 incident cancers occurred in 3,263 children who had received an x-ray, and 47 incident cancers occurred in the 6,737 children who had not received an x-ray during their hospitalization.

• Whataretheratesofcancerincidenceineachexposuregroup?
• Whatistheattributableriskofcancerduetox-rayinthisstudypopulation?Whatistheinterpretationofthisestimate?
• What is the risk ratio for the effect of exposure on the development of cancer in this study? What is the interpretation of this estimated ratio?