[Grand types of clinical studies]

This paper provides an overview of the hierarchy between the main descriptives clinical studies, with their respective weaknesses and strength.Case-reports and case-series are first described, which do not provide control groups but have been very useful in disease recognition and nosology building. Ecological studies, which are hypothesis generating at the group or population level, but which do not study individuals, and prevalence studies with one simple or a double question, which study individuals within groups but may lead to erroneous conclusions so far are risk factors are involved, are then reviewed.The overview of the descriptive studies leads to the notion of confounding factor, which may be better addressed by analytical studies.