3.2.2 Case-control studies
These focus on determining disease causation. The ‘case’ is a person who has a particular symptom or medical condition. Thus, the focus is on a group of cases which is then compared with a ‘control group’ consisting of persons not having the symptom or the medical condition. Investigations are then carried out into the previous exposure of the two groups to particular factors that are suspected of causing the symptom or condition. If the two groups differ regarding their exposure to such factors, a causal link between the symptom/condition and the factor is inferred.
A classic example of a case-control study was the important study reported by Herbst et al. (1971) and Herbst and Scully (1980), in which a clinician noticed a cluster of unusual cancers in adolescent girls. On investigation, it was found that the mothers of these girls had been treated with a hormone during pregnancy, while the mothers of the adolescent girls in the control group had not. (The particular hormone had been prescribed to prevent miscarriages in the girls’ mothers, but no one had suspected that their children might develop cancer.)