‘Searching for the causes of disease’ defines a cause as a factor without which an effect — adverse or favourable — would not have happened. Epidemiologists try to identify factors — social, biological, chemical, physical — whose presence can be shown to be constantly associated with an increase or a decrease in a disease incidence rate or risk. Three types of potential distortions threaten any observational study in epidemiology: (a) bias from uncontrolled or inadequately controlled (through adjustment methods) confounders; (b) bias from selection of subjects; and (c) bias from observation of subjects and collection of information. Bias is synonymous with systematic or constant error. In addition chance errors are always present.