ABSTRACT
Paired sampling is a technique that has proven useful in establishing cause in clinical practice and outbreak investigation. The biological properties of agents, such as pathogenicity and virulence, strains, and genetic variability, are primary determinants of the ability of an agent to cause disease. The stronger the association between a presumed causal factor and outcome, the more likely that a cause and effect relationship exists. A cause-effect relationship is more likely to exist if it can be shown that varying amounts of the suspected cause are related to varying amounts of the effect. Epidemiologic studies cannot, however, prove that a cause-effect relationship exists, only that an association exists that is unlikely to have arisen by chance alone. The way in which environment factors interact to cause disease has been referred to as the web of causation, which is another expression of the concept of multiple causality.
