spectrum in which the opposite poles represent, on one end, the nonexistence of evidence and, on the other, the evidence that is irrefutable. The challenge is therefore to determine the decision rules at play at various points along the path to ever stronger evidence.

Forms and Standards of Evidence: A Hierarchy

The decision rules will be forged by the forms of the evidence and the standards applied. In the biological sciences, both the forms and the standards take on a certain generally accepted character. The forms of evidence include biochemical data, animal studies, population studies, and individual studies. The standards of evidence relate to such issues as the consistency, strength, specificity, response of the association, and the biological plausibility. Together, these serve as a general framework for assessing health interventions.

In the case of medical care, for example, evidence is usually information from clinical experience that has met an established test of validity, with the appropriate standard determined according to the requirements of the intervention and the clinical circumstance. Typically, evidence of clinical effectiveness is conceptualized as a pyramid, in which the base of the pyramid contains the least scientifically sound type of evidence formation—professional ideas and opinions (Figure 2-1). Moving up the pyramid are ever-stronger types of evidence: case reports and case series; case-control studies and cohort studies; toward the top of the pyramid, randomized controlled studies; and, at the apex, randomized, double-blind, placebo-controlled studies—often called “the gold standard.”

While this hierarchy of evidence has been widely accepted and used in the medical community for more than a decade, its real-world application is less than perfect. It is important to look at the nature of evidence needed in the context of whether the motivating question focuses on safety, efficacy, effectiveness, or efficiency. Does the intervention under study cause harm? Does it work? Does it work in context? Is it a sound use of resources?

There is a growing recognition of the need to view evidence in a more nuanced and detailed fashion. Rather than a pyramid or hierarchy, a more comprehensive and systematic view of clinical evidence emerges when evidence is viewed through an evidence matrix, which is structured according to levels of certainty juxtaposed with levels of likely benefit, in order to provide a framework for better understanding which interventions would provide the greatest impact or greatest likelihood of impact (Pearson et al., 2003). Insight into the possible levels of impact can then be used to inform, in variable fashion, the many different types of decision-making challenges often faced in health care, such as regulation, medical coverage, guidelines, indicators used in quality care assessment, and even individual-level decisions (Teutsch, 2008). Considering the multifaceted dimensions of the application of evidence in medical care offers a sense of the complex nature of factors involved in using evidence for decision making.

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