Analytic epidemiology deals with the concept of risk. The aim is to assess the strength of a possible association with a potential risk factor. A number of specific analytical study design options can be employed. As only some risk factors can be assessed by an interventional study (e.g. a controlled clinical trial) most analytical studies are based on an observational design: a cohort or case-control studies. In a cohort study, subjects are classified by the presence or absence of exposure to a potential risk factor (e.g. silicon breast implants) and then followed for the development of the disease (e.g. systemic sclerosis). In a case-control study, a group of patients with the disease of interest (e.g. bullous pemphigoid) and a control group (e.g. without bullous pemphigoid) are compared for the proportions of the exposure of interest (e.g. number of malignancies). Risk in cohort studies is measured as relative risk and in case-control studies as Odds-ratio which is an approximation of the relative risk. A relative risk or an Odds-ratio of > 1 points to an increased risk, of<1toa decreased risk, i.e. a protective factor (Schlesselmann 1982) (Table 1).
Epidemiology of Autoimmune Skin Disorders Table 1. Interpreting Relative Risk (RR) and Odds-Ratio (OR)
RR or OR
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