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TABLE 4-5 Spectrum of Relative Concerns with Epidemiological Data

Increasing Concern

Case-control or cohort study (including registries), with small,a but statistically significant, relative risk or odds ratio of a serious adverse event

OR

Large relative risk or odds ratio of a serious adverse event that is not statistically significant

OR

Poorly conducted studies with large or significant effects

Case-control or cohort study (including registries), with moderate, statistically significant relative risk or odds ratio of a serious adverse event

OR

Moderate relative risk or odds ratio of a serious adverse event that is not statistically significant but that implies a trend

Well-conducted case-control or cohort study (including registries), with large, statistically significant relative risk or odds ratio of a serious adverse event

a Two or less is generally considered a weak association, and 3 or more is considered strong, but this is only a very general “rule of thumb” guidance and somewhat debatable.

Using Epidemiological Data on Dietary Supplements

Assessing causality from epidemiological data requires the specific study designs that are described here, for example, case-control and cohort studies. However, other types of epidemiological studies often have been used in combination with other study designs (e.g., RCTs, case reports) to draw or strengthen conclusions (GAO, 1992).

For any type of epidemiological study, the quality of the study and the analyses depends on the quality of the data. Missing data or, in the case of surveys, poor participation may cause biased results. Quality also may be affected by a conflict of interest by an author or study sponsor. Safeguards should be in place to prevent biased reporting of study results or where a conflict of interest is present. Errors in design, data collection, and analyses can also lead to poor-quality studies. Possible flaws of epidemiological studies have been well described in the medical and health care literature (Altman, 1998; Gardner et al., 1986).

Cohort Studies

Cohort studies generally evaluate a group of individuals (either prospectively or retrospectively) and estimate incidence rates of an event in



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