Another type of observational study in the field of epidemiology is the case-control study, in which researchers identify individuals with certain diseases and attempt correlate those illnesses with prior exposure to chemicals. Researchers may have to rely on individual recollections related to chemical exposures that may have occurred decades earlier. Such accounts are often unreliable and further weaken the findings. Because these studies attempt to quantify past exposures, they are classified as retrospective studies.
Other types of epidemiological studies include clinical, cohort, and ecological.
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