[Causality in epidemiology: populations, individuals, and alternative explanations].

The purpose of the present paper is to review issues concerning causal reasoning in epidemiology, with special emphasis on the relationship between observation at the individual and population level. The sequence of the three sections of the paper, causality in epidemiology, individual and population, alternative explanations, reflects the need to deal with issues of research methodology mainly derived from discussions in the frame of risk assessment, decision making, regulation and legal setting. In discussing the aforementioned topics, special attention is given to the attribution of causality to a single exposure in the presence of a plurality of potential causes. As a case study, data concerning the occurrence of hepatic and pulmonary tumors among subjects exposed to vinyl chloride are considered making reference to hearings on this topic which took place in Court.