Evidence-Based Nursing Research Guide: Evidence Levels & Types

Depending on their purpose, design, and mode of reporting or dissemination, health-related research studies can be ranked according to the strength of evidence they provide, with the sources of strongest evidence at the top, and the weakest at the bottom:

Systematic Review

• Identifies, appraises, and synthesizes all empirical evidence that meets pre-specified eligibility criteria
• Methods section outlines a detailed search strategy used to identify and appraise articles
• May include a meta-analysis, but not required (see Meta-Analysis below)

Meta-Analysis

• A subset of systematic reviews: uses quantitative methods to combine the results of independent studies and synthesize the summaries and conclusions
• Methods section outlines a detailed search strategy used to identify and appraise articles; often surveys clinical trials
• Can be conducted independently, or as a part of a systematic review
• All meta-analyses are systematic reviews, but not all systematic reviews are meta-analyses

Evidence-Based Guideline

• Provides a brief summary of evidence for a general clinical question or condition
• Produced by professional health care organizations, practices, and agencies that systematically gather, appraise, and combine the evidence
• Click on the 'Evidence-Based Care Sheets' link located at the top of the CINAHL screen to find short overviews of evidence-based care recommendations covering 140 or more health care topics.

Meta-Synthesis or Qualitative Synthesis (Systematic Review of Qualitative or Descriptive Studies)

• a systematic review of qualitative or descriptive studies, low strength level

Randomized Controlled Trial

• Experiment where individuals are randomly assigned to an experimental or control group to test the value or efficiency of a treatment or intervention

Non-Randomized Controlled Clinical Trial (Quasi-Experimental)

• Involves one or more test treatments, at least one control treatment, specified outcome measures for evaluating the studied intervention, and a bias-free method for assigning patients to the test treatment

Case-Control or Case-Comparison Study (Non-Experimental)

• Individuals with a particular condition or disease (the cases) are selected for comparison with individuals who do not have the condition or disease (the controls)

Cohort Study (Non-Experimental)

• Identifies subsets (cohorts) of a defined population
• Cohorts may or may not be exposed to factors that researchers hypothesize will influence the probability that participants will have a particular disease or other outcome
• Researchers follow cohorts in an attempt to determine distinguishing subgroup characteristics