Evidence Based Practice Guide for Nursing Students: Levels of Evidence

Hierarchical order of the types of resources based on highest level of credibility.

From Library of Health Sciences-Chicago, University of Illinois at Chicago, http://ebp.lib.uic.edu/pharmacy/node?page=6

Secondary Literature

The practice and teaching of Evidence Based Medicine has outcome products which help the health care provider and consumer keep up with the medical literature and assess the evidence. This secondary literature synthesizes, filters, and evaluates the primary research literature. Dissemination and incorporation of valid clinical research findings into medical practice is the ultimate goal.

There are several types of secondary literature:

• Systematic Reviews
• Meta-analyses
• Evidence Based Practice Guidelines
• Critically Appraised Topics (CATs)
• Decision Analyses/Decision Tools
• Consensus Development Reports

Systematic Reviews

Systematic Reviews are NOT the same as traditional reviews.  Systematic Reviews start with a clearly articulated question and use explicit and rigorous methods.  They include a description of how primary data sources and articles are found and selected for inclusion.  They critically appraise the found evidence by assessing their validity, reliability, and other measures of quality.  Finally, Systematic Reviews combine and analyze the results of this evidence to synthesize it into an evidence based review. 

When searching PubMed try the Systematic Review - Article Types filter

Or adding one of these to your search

• Review [PT] AND medline [TW]
• (Quantitative OR Systematic OR Methodologic) AND (Review OR Overview)

Meta-Analyses

Meta-analyses are similar to Systematic Reviews, but they include a specific methodologic and statistical technique for combining quantitative data.  They provide an evidence based overview of multiple studies with combined quantitative analysis.

When searching PubMed try the Meta-Analysis - Article Types filter

Or adding one of these to your search

• Meta-analysis [PT]
• meta-anal* [TW] OR
• metaanal* [TW]

Attribution Statement: University of Illinois Chicago. Library of the Health Sciences. Evidence Based Medicine. https://researchguides.uic.edu/ebm. Used under a CC BY-NC license.

Study Design Concepts

Studies can be descriptive (or observational), such as:

• Case Reports [MeSH Publication Type] - "Clinical presentations that may be followed by evaluative studies that eventually lead to a diagnosis."

• Case Series - Descriptions of groups of patients with a disease.

• Case-Control Studies [MeSH Term] - "Comparisons that start with the identification of persons with the disease or outcome of interest and a control (comparison, referent) group without the disease or outcome of interest. The relationship of an attribute is examined by comparing both groups with regard to the frequency or levels of outcome over time."

• Cohort Studies [MeSH Term] - "Studies in which subsets of a defined population are identified. These groups may or may not be exposed to factors hypothesized to influence the probability of the occurrence of a particular disease or other outcome. Cohorts are defined populations which, as a whole, are followed in an attempt to determine distinguishing subgroup characteristics."

Or evaluative (interventional), such as a:

• Clinical Trial [MeSH Publication Type] - "A work that reports on the results of a clinical study in which participants are assigned to receive one or more interventions so that researchers can evaluate the interventions on biomedical or health-related outcomes. The assignments are determined by the study protocol. Participants may receive diagnostic, therapeutic, or other types of interventions."
• Controlled Clinical Trial [MeSH Publication Type] - "A work that reports on a clinical trial involving 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. The treatment may be drugs, devices, or procedures studied for diagnostic, therapeutic, or prophylactic effectiveness. Control measures include placebos, active medicine, no-treatment, dosage forms and regimens, historical comparisons, etc. When randomization using mathematical techniques, such as the use of a random numbers table, is employed to assign patients to test or control treatments, the trial is characterized as a RANDOMIZED CONTROLLED TRIAL."

• Randomized Controlled Trial [MeSH Publication Type] - "A work that reports on a clinical trial that involves at least one test treatment and one control treatment, concurrent enrollment and follow-up of the test- and control-treated groups, and in which the treatments to be administered are selected by a random process, such as the use of a random-numbers table."

Studies can also be time-based:

• Follow-up Studies [MeSH Term] - "Studies in which individuals or populations are followed to assess the outcome of exposures, procedures, or effects of a characteristic, e.g., occurrence of disease."
• Longitudinal Studies [MeSH Term] - "Studies in which variables relating to an individual or group of individuals are assessed over a period of time."

Attribution Statement: University of Illinois Chicago. Library of the Health Sciences. Evidence Based Medicine. https://researchguides.uic.edu/ebm. Used under a CC BY-NC license.

Filtered Source:

Systematic Reviews, Evidence Summaries, Structured Abstracts, and evidence-based clinical practice guidelines that have been written by experts in the field who have critically appraised studies and summarize the results.

Using filtered sources of information, such as Cochrane Database of Systematic Reviews, Joanna Briggs Institute, National Guideline Clearinghouse, and ECRI Evidence Summaries will reduce the time needed to select and appraise studies during clinical decision-making.

Unfiltered Sources:

Randomised controlled trials, quantitative studies and cohort studies are primary literature.

These are found in peer reviewed journals and are indexed in bibliographic databases such as PubMed and CINAHL. Critical appraisal must be done when using studies to determine risk of bias, validity of the studies and relevance to the PICO question.

Landmark studies are research that have significantly influenced an area of nursing, medicine or allied health. Frequently, these are the first studies that identified a specific disorder, treatment or procedure.

Sometimes these studies are well known, such as the Framingham studies which were the first to make the association between lifestyle and risk factors for disorders such as heart disease. At the time of a study, it is not known whether a study will be significant or will be the basis of a series of related studies. For that reason, you can not search for a landmark study.

The best way to recognize a study is to look at the bibliography of several articles on the same subject. If the same article is cited in most or all of the articles, it is very likely that you have found a landmark study. Another way to check to see if the article you have found is a landmark study is to do a Cited Reference Search in Web of Science. Landmark studies will be cited by hundreds of other articles.