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Complementary and Alternative Medicine in the United States
TABLE 3-4 Hierarchy of Evidence
Level
Emphasis on Efficacy
Emphasis on Effectiveness
I
Systematic Review (e.g., meta-analysis of Several Well-Controlled Randomized Trials—consistent results
Systematic Review (e.g., meta-analysis) of Several Well-Designed Outcome Studies or “Effectiveness RCTs”—consistent results
II
Single Well-Controlled Randomized Trial
Single Well-Designed Outcomes Study or “Effectiveness RCT”
III
Consistent Findings from Multiple Cohort, Case-Control, or Observational Studies
IV
Single Cohort, Case-Control, or Observational Study
V
Uncontrolled Experiment, Unsystematic Observation, Expert Opinion, or Consensus Judgments
SOURCE: IOM, 2001.
In general an RCT is the preferred study design if the issue is establishing treatment efficacy.
More studies are better than fewer studies, therefore a meta-analysis of multiple good RCTs is better than one good RCT.
Other study designs can provide evidence of efficacy or effectiveness.
Meta-analysis of multiple non-RCT studies is better than one non-RCT study. Meta-analysis of multiple non-RCT studies may or may not be better than one good RCT; it depends on the details of the studies and the specific question being asked.
If the question is treatment effectiveness, then some features of the typical RCT (stringent inclusion/exclusion criteria; treatment given in high-quality, high-volume clinical sites; detailed, frequent patient follow-up; etc.) create problems in generalizing findings to routine practice settings.
Other study designs, including observational studies or “effectiveness RCTs,” may provide evidence that is at least equally compelling as that provided by an “efficacy RCT.”
Effect size is another consideration that must be taken into account along with features of study design when one weighs the strength of evidence for a particular therapy. Treatments with clear, dramatic, positive effects in small or less well-controlled studies may be deemed “efficacious” sooner than treatments with more modest effects.
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