Introduction
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:
Levels of Evidence Pyramid: Systematic Reviews
Further reading
Cochrane Handbook for Systematic Reviews of Interventions
*updated 2019
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.
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:
Levels of Evidence Pyramid: Critically-Appraised Topics
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:
Levels of Evidence Pyramid: Critically-Appraised Articles
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:
CINAHL Search Help: Randomized Controlled Trials
MEDLINE Search Help: Randomized Controlled Trials
Further reading
CONSORT 2010 Statement: updated guidelines for reporting parallel group
randomised trials
CONSORT 2010 Explanation and Elaboration: updated guidelines for reporting
parallel group randomised trials
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:
CINAHL Search Help: Cohort Studies
MEDLINE Search Help: Cohort Studies
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:
CINAHL Search Help: Case Studies
MEDLINE Search Help: Case Studies
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:
Levels of Evidence Pyramid: Background Information & Expert Opinion
