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Evidence-Based Research: Evidence Types


Not all evidence is the same, and appraising the quality of the evidence is part of evidence-based practice research. The hierarchy of evidence is typically represented as a pyramid shape, with the smaller, weaker and more abundant research studies near the base of the pyramid, and systematic reviews and meta-analyses at the top with higher validity but a more limited range of topics.

Several versions of the evidence pyramid have evolved with different interpretations, but they are all comprised of the types of evidence discussed on this page. Walden's Nursing 6052 Essentials of Evidence-Based Practice class currently uses a simplified adaptation of the Johns Hopkins model.

Evidence Levels:

Level I: Experimental, randomized controlled trial (RCT), systematic review RTCs with or without meta-analysis

Level II: Quasi-experimental studies, systematic review of a combination of RCTs and quasi-experimental studies, or quasi-experimental studies only, with or without meta-analysis

Level III: Nonexperimental, systematic review of RCTs, quasi-experimental with/without meta-analysis, qualitative, qualitative systematic review with/without meta-synthesis  (see Daly 2007 for a sample qualitative hierarchy) 

Level IV: Respected authorities’ opinions, nationally recognized expert committee or consensus panel reports based on scientific evidence

Level V: Literature reviews, quality improvement, program evaluation, financial evaluation, case reports, nationally recognized expert(s) opinion based on experiential evidence

Systematic review

What is a Systematic Review?

A systematic review is a type of publication that addresses a clinical question by analyzing research that fits certain explicitly-specified criteria. The criteria for inclusion is usually based on research from clinical trials and observational studies. Assessments are done based on stringent guidelines, and the reviews are regularly updated. These are usually considered one of the highest levels of evidence and usually address diagnosis and treatment questions.

Benefits of Systematic Reviews

Systematic reviews refine and reduce large amounts of data and information into one document, effectively summarizing the evidence to support clinical decisions. Since they are typically undertaken by a entire team of experts, they can take months or even years to complete, and must be regularly updated. The teams are usually comprised of content experts, an experienced searcher, a bio-statistician, and a methodologist. The team develops a rigorous protocol to thoroughly locate, identify, extract, and analyze all of the evidence available that addresses their specific clinical question.

As systematic reviews become more frequently published, concern over quality led to the PRISMA Statement to establish a minimum set of items for reporting in systematic reviews and meta-analyses.

Many systematic reviews also contain a meta-analysis.

What is a Meta-Analysis?

Meta-analysis is a particular type of systematic review that focuses on selecting and reviewing quantitative research. Researchers conducting a meta-analysis combine the results of several independent studies and reviews to produce a synthesis where possible. These publications aim to assist in making decisions about a particular therapy.

Benefits of Meta-Analysis

A meta-analysis synthesizes large amounts of data using a statistical examination. This type of analysis provides for some control between studies and generalized application to the population.

To learn how to find systematic reviews in the Walden Library, please see the Levels of Evidence Pyramid page:


Further reading

Guidelines & summaries

Practice Guidelines

A practice guideline is a systematically-developed statement addressing common patient health care decisions in specific clinical settings and circumstances.  They should be valid, reliable, reproducible, clinically applicable, clear and flexible. Documentation must be included and referenced. Practice guidelines may come from organizations, associations, government entities, and hospitals/health systems.

ECRI Guidelines Trust

Best Evidence Topics

Best evidence topics are sometimes referred to as Best BETs. These topics are developed and supported for situations or setting when the high levels of evidence don't fit or are unavailable. They originated from emergency medicine providers' need to conduct rapid evidence-based clinical decisions.

Critically-Appraised Topics

Critically-appraised topics are a standardized one- to two-page summary of the evidence supporting a clinical question. They include a critique of the literature and statement of relevant results. They can be found online in many repositories.


To learn how to find critically-appraised topics in the Walden Library, please see the Levels of Evidence Pyramid page:

Critically-Appraised Articles

Critically-appraised articles are individual articles by authors that evaluate and synopsize individual research studies. ACP Journal Club is the most well known grouping of titles that include critically appraised articles.


To learn how to find critically-appraised articles in the Walden Library, please see the Levels of Evidence Pyramid page:

Randomized controlled trial

A randomized controlled trial (RCT) is a clinical trial in which participants are randomly assigned to either the treatment group or control group. This random allocation of participants helps to reduce any possible selection bias and makes the RCT a high level of evidence. Having a control group, which receives no treatment or a placebo treatment, to compare the treatment group against allows researchers to observe the potential efficacy of the treatment when other factors remain the same. Randomized controlled trials are quantitative studies and are often the only studies included in systematic reviews.

To learn how to find randomize controlled trials, please see our CINAHL & MEDLINE help pages:

Cohort study

A cohort study is an observational longitudinal study that analyzes risk factors and outcomes by following a group (cohort) that share a common characteristic or experience over a period of time.

Cohort studies can be retrospective, looking back over time at data that has already been collected, or can be prospective, following a group forward into the future and collecting data along the way.

While cohort studies are considered a lower level of evidence than randomized controlled trials, they may be the only way to study certain factors ethically. For example, researchers may follow a cohort of people who are tobacco smokers and compare them to a cohort of non-smokers looking for outcomes. That would be an ethical study. It would be highly unethical, however, to design a randomized controlled trial in which one group of participants are forced to smoke in order to compare outcomes.


To learn how to find cohort studies, please see our CINAHL and MEDLINE help pages:

Case-controlled studies

Case-controlled studies are a type of observational study that looks at patients who have the same disease or outcome. The cases are those who have the disease or outcome while the controls do not. This type of study evaluates the relationship between diseases and exposures by retrospectively looking back to investigate what could potentially cause the disease or outcome.


To learn how to find case-controlled studies, please see our CINAHL and MEDLINE help pages:

Background information & expert opinion

Background information and expert opinion can be found in textbooks or medical books that provide basic information on a topic. They can be helpful to make sure you understand a topic and are familiar with terms associated with it.


To learn about accessing background information, please see the Levels of Evidence Pyramid page: