KEY

Investigator at beginning of study

? To be determined

At the beginning of a retrospective cohort study, the investigator is aware of an exposure to a hazard that occurred at some time in the past, sufficiently long ago that if disease were to have occurred, it should by now be evident. A cohort that includes the exposed individuals is identified, and the health histories of all members explored to identify the presence or absence of disease in all individuals, and the difference in rate between exposed and non-exposed.

Many of the studies of association between environmental exposures and disease have been of this type. A cohort of individuals who lived or worked in an area but had different experiences of exposure/non-exposure to a chemical will be identified. Their health histories in the intervening years will be examined to identify differences, if any, in their rates of disease.