A case control-study was conducted to examine the different risk factors (e.g. parent's education, family income, ethnicity, smoking, herbicides and solvents exposure, etc.) associated with leukemia among children. Investigators recruited cases of acute lymphoblastic leukemia among children in several hospitals in the Bay area. Controls were selected in the same hospitals as the cases. Controls included children diagnosed for other conditions or diseases than acute lymphoblastic leukemia and blood-related diseases.

a. What is the purpose of a control group in a case-control study?

b. What are the advantages and disadvantages of selecting this hospital-based control group?

c. The investigators decided to match the cases and controls for gender. Explain the reasons behind this decision in this particular study.

2. How is a case-control study different from a retrospective cohort study?