limitations. This does not preclude checking original sources where feasible, but helps in judging the credibility of secondary sources. In general, one should also always rule out any conflicts of interest in secondary compilations of evidence. Parallel evidence is particularly helpful in answering “What” and “How” questions. Table 5-9 provides examples of how this type of evidence might be used. Also see Chapter 2 for examples of parallel evidence, including activities surrounding tobacco and alcohol use and HIV/AIDS.

Expert Knowledge

Expert knowledge includes the views of professionals with expertise in a particular field of practice or inquiry, such as clinical practice or research methodology. Expert opinion may refer to one person’s views or to the consensus view of a group of experts. Consensus views are sometimes based on systematic reviews or other forms of evidence synthesis but still require interpretation or judgment about the evidence that is collected, and may require drawing conclusions in the absence of any or enough relevant evidence. Expert consensus and expert opinion should be appraised based on the credentials and experience of the experts involved, supporting documentation, the transparency and rigor of the consensus process, and ruling out of any conflicts

TABLE 5-9 Types of Parallel Evidence and Examples of Their Uses

Type of Evidence

Questions That Can Be Addressed

Specific Applications

Research Evidence on Effects of Parallel Interventions

Given the existing evidence on the effectiveness of tobacco and alcohol taxes, would soda taxes be similarly effective in reducing obesity on a large scale? (“What” question)

Intervention impact or effectiveness studies showing that strategies to influence public behaviors work

Parallel Research on Legal Issues

Given the constitutional issues involved in restricting free speech, what grounds have been used to justify controls on advertising? (“What” question)

Content analysis of relevant cases to identify arguments that have been advanced for and against such restrictions and how these arguments have been resolved

Parallel Research on Implementation Process or Policy Development

Given that eating and physical activity are individual behaviors but are affected by policies and programs in the broader community, what are some precedents for environmental and policy approaches that impact personal behavior, and how were they achieved and justified? (“What” question)

Case studies of effects of populationwide interventions on nutrition, physical activity, obesity, or cardiovascular disease or obesity in other countriesa


Given that obesity is a populationwide problem for which many of the drivers are a part of the social fabric, what can be learned from approaches used in other public health efforts of similar scope and complexity? (“How” question)

Retrospective case analysis of successes related to other complex public health issues that have been addressed by policies that have led to social changea

a See Economos et al., 2001; Eriksen, 2005; Kersh and Morone, 2002.

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