Notions d'épidémiologie utiles en dermatologie

Epidemiology was initially defined as the study of the distribution of the diseases among the populations (descriptive epidemiology), as well as the identification of the factors which determine this distribution (explicative epidemiology). Now, epidemiology is also interested in the evaluation of the diagnostic and therapeutic procedures and in that of the predictive factors. Epidemiology serves as a guide for public health and medical decisions. The main indicators used in descriptive studies are the prevalence, incidence and mortality rates. Data are collected by systematic reporting or specific surveys. The aim of analytic epidemiology is to ascertain a cause-effect relationship between an exposure and a disease. The strength of the association is estimated by the relative risk or the odds ratio. Other risk estimates are the attributable risk and the etiologic fraction. Interpretation of analytic studies needs to evaluate the role of chance (statistically significant association between the exposure and the disease), the likelihood that the results reflect potential biases or confounding factors, and the causality assessment. Assessing medical strategies implies using the notions of sensitivity, specificity and likelihood ratios. These allow determining the probability a patient has to be hill according to his clinical characteristics and to the results of the examination. Assessing prognostic factors allows estimating the probability of the clinical evolution according to the patients' characteristics.