Epidemiology is the study of the occurrence of disease in groups of people, and the application of that information to the control of health problems. Epidemiological studies look at large numbers of people (a population, as opposed to an individual) to study the association between exposure to a substance or event with the occurrence of an identified disease. Most health regulations today are based on epidemiological studies.
There are some complications with epidemiological studies. Many detailed factors about substance exposure, including the duration of exposure and intensity of exposure, can affect the conclusions of an epidemiological study. According to the National Institute of Environmental Health Sciences, this information may not be available or reliable because:
People may not know what they have been exposed to or the amount of their exposure.
People find it very difficult to recall details of exposures that occurred many years ago.
In an epidemiological study, it can be difficult to detect small differences in the rates of common diseases with precision. When a very large number of people may be reliably affected by a common disease, relatively small differences cannot be identified by epidemiological studies.