Case Control Study (10 points): A group of people with disease X (i.e., the "case" group) is compared to a group of people who do not have X ("controls"). For each case, one to three controls are selected. The controls are matched to the case on factors which may be relevant to the disease, such as sex, age, SES, and smoking habits. These matching factors, "confounders", may increase or decrease the probability of getting X, so must be controlled.
In a typical study, "cases" are people who enter the hospital for treatment of disease X, between Jan 1 and Jan 31, 2026. Controls are people who entered the hospital during that interval, but not to receive treatment for X. Two controls for each case are matched on age and sex. The fewer the matching parameters, the easier control selection is. The person doing the study thinks that something specific (Y) is causing X, and perhaps Y is a chemical, overexposure to a specific chemical, a drug or a certain food. The object of the study is to validate the hypothesis that Y causes X. Cases and controls are sorted according to exposure to the Y. If the exposed group has a significantly higher incidence of X then the hypothesis may be true.
Analysis: Place data in a 2x2 box, label boxes a, b, c and d and calculate the odds ratio, OR = ac/bd.