In order to investigate the relationship between dietary aflatoxins and hepatocellular carcinoma (HCC), all cases of liver cancer developed in the past five years and liver cancer-free controls selected from a pre-existing study cohort at the time of diagnosis were included in the analysis. The exposure of interest, aflatoxins, was assessed from dietary recall questionnaires collected from the subjects at the beginning of the study. a. Describe a risk measurement that you think would be most appropriate to compare the groups, and justify your choice.
b. Describe the study as to its particular sampling method.