Designing case-control studies: the validity of case-control studies hinges upon 3 aspects of the study design, selecting cases 2) selecting controls 3) measuring the exposure of interest. Case-control study design--selecting cases: explicit criteria to define case/control, else lead to miscategorization, erroneous/biased estimate, cases should represent all possible cases. This ensures that cases selected do not represent extreme/atypical cases of outcome or exposure & is simpler to find matched controls: cases should be new (incident), not prevalent ones. Prevalent cases are more likely to contribute bias due to factors related to disease duration: sample: li et al, 2009 bmi and risk, age of onset, and survival in pancreatic cancer. 2 www. notesolution. com: explicit criteria should be used to define cases & controls. Case: confirmed pancreatic ductal adenocarcinoma (controls are the opposite but at risk): cases selected for inclusion should be representative of all possible cases.