An observational study method that selects people with a disease and those without a disease to investigate their exposure to a risk factor and its degree respectively, so as to determine whether the exposure risk factor is related to a disease and the degree of association. Drawbacks: (Tobacco case as an example)

• Retrospective: We know the subject has cancer, and in that context we have to go back and figure out why.
• Reversed logic of probability: What these data tell us is the probability that cancer patients are smokers, not the probability that smokers get cancer. For those who want to know if they should smoke, the probability of cancer in smokers is the probability they really care about.
• Recall bias: While the interviewers may be blinded to cancer vs. control, the patients themselves know whether or not they had cancer. Having already gone through this event can affect their recollections.
• Selection bias: Hospitalized cancer patients (aka those on record as having cancer and those specifically hospitalized for it) are not necessary representative of the population at large or the smoking population at large.