Subject Librarian
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Levels of Evidence-based Resources
You will encounter different levels of evidence in research or decision-making. Evidence can come from pre-appraised (filtered) sources, like systematic reviews, guidelines, and summaries, or from non-appraised (unfiltered) sources, like individual journal articles that you’ll need to evaluate yourself.
This guide explains each level of evidence and highlights key databases and resources to help you start your search.
Summaries & Guidelines
Summaries and evidence-based guidelines draw on the best available evidence across multiple levels to present a broad overview and outline management options for a given health problem.
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Guideline Central Guideline SummariesSite with a wide range of guidelines from many jurisdictions.
Preappraised Research
SYNOPSES OF SYNTHESES AND STUDIES
These are short, expert-written summaries of systematic reviews or primary studies. They usually include a quality rating or critical appraisal so you can quickly judge the reliability of the evidence.
SYNTHESES
Syntheses combine findings from multiple primary studies to provide an overall understanding of a research question. Common examples include systematic reviews, scoping reviews, health technology assessments, and meta-analyses. These critically assess the included studies for quality and risk of bias.
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PubMed Systematic Review SubsetCombine 'systematic [sb]' with your search terms using 'AND' . This will filter your results to possible systematic reviews.
Non Preappraised Research
PRIMARY STUDIES
Primary studies report new research findings. Their quality hasn’t been appraised, so critical evaluation is essential.
These databases focus on high-quality primary studies, especially randomized controlled trials, making it easier to find strong study designs.
Unfiltered databases include all publication types (primary research studies, journal articles, letters, and commentaries). Because quality varies, study design filters or clinical queries can help you focus on the most relevant study designs.
Meta Search Engines (All Evidence Levels)
These search across multiple evidence sources at once, helping you quickly find guidelines, systematic reviews, and primary studies in a single search.