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Strength of the Evidence
Level I Meta-analysis, systematic review, randomized controlled trials (RCTs)
Level II Experimental or quasi-experimental studies, cohort studies
Level III Non-experimental, qualitative studies or case-control studies
Level IV Case series, case reports
Level V Expert opinion, animal and in vitro studies
Quality of the Evidence
High (A)  
Scientific Consistent results with sufficient sample size, adequate control, and definitive conclusions, consistent recommendations based on extensive literature review that includes thoughtful reference to scientific evidence.
Summative reviews Well-defined, reproducible search strategies; consistent results with sufficient numbers of well-defined studies; criteria-based evaluation of overall scientific strength and quality of included studies; definitive conclusions.
Experiential Expertise is clearly evident.
Good (B)  
Scientific Reasonably consistent results, sufficient sample size, some control, with fairly definitive conclusions; reasonably consistent recommendations based on fairly comprehensive literature review that includes some reference to scientific evidence.
Summative reviews Reasonably thorough and appropriate search; reasonably consistent results with sufficient numbers of well-defined studies; evaluation of strengths and limitations of included studies; fairly definitive conclusions.
Experiential Expertise seems to be credible.
Low quality (C)  
Scientific Little evidence with inconsistent results, insufficient sample size, conclusions cannot be drawn.
Summative reviews Undefined, poorly defined or limited search strategies; insufficient evidence with inconsistent results; conclusions cannot be drawn.
Experiential Expertise is not discernable or is dubious.


* Adapted from Newhouse (2006, JONA, 36:7/8) and ACSO (