Figure 2.1  shows that  systematic reviews  are at the pinnacle of the hierarchy (Level I), regardless of the type of question, because the strongest evidence comes from careful syntheses of multiple studies. The next highest level (Level II) depends on the nature of inquiry. For Therapy questions regarding the efficacy of an intervention (What works best for improving health outcomes?), individual RCTs constitute Level II evidence (systematic reviews of multiple RCTs are Level I). Going down the “rungs” of the evidence hierarchy for Therapy questions results in less reliable evidence—for example, Level III evidence comes from a type of study called quasi-experimental. In-depth qualitative studies are near the bottom, in terms of evidence regarding intervention effectiveness. (Terms in  Figure 2.1  will be discussed in later chapters.)
FIGURE 2.1
Evidence hierarchy: levels of evidence.
For a Prognosis question, by contrast, Level II evidence comes from a single prospective cohort study, and Level III is from a type of study called case control (Level I evidence is from a systematic review of cohort studies). Thus, contrary to what is often implied in discussions of evidence hierarchies, there really are multiple hierarchies. If one is interested in best evidence for questions about Meaning, an RCT would be a poor source of evidence, for example. We have tried to portray the notion of multiple hierarchies in  Figure 2.1 , with information on the right indicating the type of individual study that would offer the best evidence (Level II) for different questions. In all cases, appropriate systematic reviews are at the pinnacle. Information about different hierarchies for different types of cause-probing questions is addressed in  Chapter 9 .
Of course, within any level in an evidence hierarchy, evidence quality can vary considerably. For example, an individual RCT could be well designed, yielding strong Level II evidence for Therapy questions, or it could be so flawed that the evidence would be weak.
Thus, in nursing, best evidence refers to research findings that are methodologically appropriate, rigorous, and clinically relevant for answering persistent questions—questions not only about the efficacy, safety, and cost-effectiveness of nursing interventions but also about the reliability of nursing assessment tests, the causes and consequences of health problems, and the meaning and nature of patients’ experiences. Confidence in the evidence is enhanced when the research methods are compelling, when there have been multiple confirmatory studies, and when the evidence has been carefully evaluated and synthesized.
Of course, there continue to be clinical practice questions for which there is relatively little research evidence. In such situations, nursing practice must rely on other sources—for example, pathophysiologic data, chart review, quality improvement data, and clinical expertise. As Sackett and colleagues (2000) have noted, one benefit of the EBP movement is that a new research agenda can emerge when clinical questions arise for which there is no satisfactory evidence.