A member of Congress who chairs the congressional committee that funds Medicare is interested in the effectiveness of a new weight control medication that the Medicare program has just begun covering. Of particular interest is the change in the health of recipients of the new drug after six months and after twelve months. The Center for Medicare and Medicaid Services has retained you to conduct an evaluation of the effectiveness of the medication when given to Medicare patients and show report that can be used to report back to Congress. Identify some of the threats to validity that might arise from the design of such an evaluation and how you might overcome these threats.