Level I represents the most robust level of evidence. Many databases include filters that can help you limit your search results to different levels of evidence.
Levels of Evidence:
Levels of Evidence (adapted from Sackett, D.L., Rosenberg, W.M., Muir Gray, J.A., Haynes, R.B. & Richardson, W.S. (1996). Evidence-based medicine: What it is and what it isn’t. British Medical Journal, 312, 71-72).
Level I: Systematic reviews, meta-analyses, randomized controlled trials
Level II: Two groups, nonrandomized studies (e.g., cohort, case-control)
Level III: One group, nonrandomized (e.g., before and after, pretest and posttest)
Level IV: Descriptive studies that include analysis of outcomes (single subject design, case series)
Level V: Case reports and expert opinion that include narrative literature reviews and consensus statements
AOTA EBP Project, GUIDELINES TO CRITICALLY APPRAISED PAPER (CAP) WORKSHEET, EVIDENCE EXCHANGE
Also see : http://libguides.rutgers.edu/c.php?g=337282&p=2269540
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