BOX 3.1 Definition of Levels of Evidence for Epidemiological Associations

Level of Evidence



• Epidemiological findings across a large number of well-designed studies are consistent.

• A dose-response relationship has been demonstrated.

• Mechanisms are biologically plausible.

• Laboratory evidence is supportive.


• Epidemiological evidence is either inconsistent or not extensive enough to make definitive judgment.

• Laboratory or mechanistic evidence is supportive.


• Epidemiological findings are supportive but limited in quantity, quality, or consistency.

• Supportive mechanistic or laboratory evidence may or may not be available.

• Supportive evidence from other disciplines may be available.


SOURCE: Adapted from (World Cancer Research Fund and American Institute for Cancer Research, 1997).

of a group of individuals who share a common experience within a defined time period and monitoring of those individuals forward in time for the development of disease (Mausner and Kramer, 1985). These studies have been essential to providing an understanding of the links between health-related behaviors and health outcomes. Other evidence comes from case-control studies in which investigators identify a group of patients with a particular cancer (cases) and a group of patients without cancer (controls) and then compare the histories of the cases and controls to determine the extent to which each was exposed to the intervention of interest.


Tobacco is the scourge of public health. In the United States alone, tobacco use, primarily in the form of cigarette smoking, causes more than 440,000 premature deaths from cancer and other causes each year and is responsible for approximately 30 percent of all cancer-related deaths (ACS, 2002b). Worldwide, the numbers are even more staggering. The World Health Organization estimated that in 1998 there would be approximately 4 million deaths linked to tobacco use worldwide (World Health Organization, 1999). By 2020, this number is expected to at least double.

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