TABLE 3

Evidence Levels

LevelDefinition
1RCTs or meta-analyses of multiple clinical trials with substantial treatment effects
2RCTs with smaller or less significant treatment effects
3Prospective, controlled, nonrandomized, cohort studies
4Historic, nonrandomized, cohort or case-control studies
5Case series: patients compiled in serial fashion, lacking a control group (excluded from review)
6Animal studies or mechanical model studies (excluded from review)
7Extrapolations from existing data collected for other purposes, theoretical analyses (excluded from review)
8Rational conjecture (common sense); common practices accepted before evidence-based guidelines (excluded from review)