Literature review designs
Narrative Review
Systematic Review
Meta-analysis
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Literature review
• Defined as a systematic, explicit, and
reproducible way of identifying, evaluating,
and interpreting all of the research findings
and scholarly work available on a topic
– A high-quality review is not haphazard
– Ideally, all of the existing work should be
included
• Considered descriptive or observational
Evidence-based Chiropractic
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© 2006-7
The three types of
literature review designs
Narrative Review
Systematic Review
Meta-analysis
Evidence-based Chiropractic
Selective review of the literature that broadly covers
a specific topic.
Does not follow strict systematic methods to locate
and synthesize articles.
Utilizes exacting search strategies to make certain
that the maximum extent of relevant research has
been considered.
Original articles are methodologically appraised and
synthesized.
Quantitatively combines the results of studies that
are the result of a systematic literature review.
Capable of performing a statistical analysis of the
pooled results of relevant studies.
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Narrative reviews
• Summarize in general what is in the
literature on a given topic
– Often written by experts in a given field
– A good source for background information
• Do not follow strict systematic methods
like the other literature review designs
– Therefore, they are prone to bias
– Lower in the hierarchy of evidence
Evidence-based Chiropractic
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© 2006-7
Narrative reviews (cont.)
• Authors like to write them because they
are relatively easy to carry out and
compose
• Practitioners like to read them because
they are easier to comprehend than more
complex designs
– Do not require a lot of background knowledge
to understand the message
Evidence-based Chiropractic
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Narrative reviews
are prone to bias
• They do not employ many of the
safeguards needed to control against bias
– Authors may be selective as to which articles
are included
– They may include articles that support their
hypothesis and exclude those that do not
– Rigorous appraisal methods are not used to
evaluate included articles
Evidence-based Chiropractic
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Prone to bias (cont.)
• During the literature search
– Authors have their own opinions on the topic
and may try to find studies that support their
viewpoint and overlook conflicting studies
• During the synthesis of the literature
– The approach to analyzing the collected
information is often subjective and
disorganized
Evidence-based Chiropractic
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© 2006-7
Prone to bias (cont.)
• In the discussion and conclusion
– The authors’ opinions may be mixed together
with evidence
– Authors may simply count the number of
studies on each side of an issue and then
espouse the view presented by the majority
without considering the strength of each study
• e.g., quality, research design used, the effect size,
and sample size
Evidence-based Chiropractic
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Selection bias in
narrative reviews
• a.k.a., reference bias
• Occurs when authors choose articles that
support their own conclusions and exclude
articles with conflicting views
– Results in an erroneous representation of the
literature
– The review may lead undiscerning readers
astray
Evidence-based Chiropractic
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Systematic reviews
• Use strict methods to locate, appraise and
synthesize all research on a topic
– Similar to narrative reviews, but with improved
procedural quality
• Designed to answer specific clinical
questions where several primary studies
exist
– Thus, a good source of clinical evidence
Evidence-based Chiropractic
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Systematic reviews (cont.)
• Articles are evaluated using appraisal
instruments
– In an attempt to achieve impartiality
– More reproducible
• Dissimilarities between the findings of
studies are investigated
• Multiple reviewers are usually involved
– Any disagreements are resolved
Evidence-based Chiropractic
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Systematic reviews (cont.)
•
The search and selection criteria for
articles are well defined
– Multiple databases should be searched
– Explicit inclusion criteria
•
The results of the included studies are
qualitatively or quantitatively synthesized
– Qualitative – written information is merged
– Quantitative – data are merged
Evidence-based Chiropractic
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Systematic reviews (cont.)
•
•
Because of strict methodology and
thoroughness, conclusions are typically
less biased than narrative reviews
Nevertheless, they may still be
influenced by the authors’ opinions
– Still a potential for selection bias
– Criteria may be applied differently when
appraising included studies
– Therefore, must be critically appraised
Evidence-based Chiropractic
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Systematic reviews (cont.)
•
No widely accepted method exists for
assessing the validity of studies
– i.e., the process is a judgment call
– Thus, reviewers sometimes disagree
•
Information derived from a systematic
review may be too narrow to completely
answer a specific clinical question
– Given that they are designed to answer
focused questions about patient care
Evidence-based Chiropractic
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Differences between narrative and systematic reviews
Feature
Narrative Review
Topic Typically broad-scoped
Data sources and The search strategy and
search strategy databases that were used may
not be provided
Authorship A recognized expert(s) on the
topic
Article selection Typically not specified
criteria
Searching May be extensive, intended to
locate literature on the topic
area in question
Appraisal of Indefinite, may be variable
included articles
Synthesis A qualitative summary is
usually provided
Inferences Sometimes evidence-based
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Evidence-based Chiropractic
Systematic Review
Focused research question
The search strategy is explicit and
comprehensive with a list of all
databases that were utilized
A team of experts having methodologic
and clinical expertise
Consistently applied inclusion and
exclusion criteria
Extensive, intended to locate all primary
studies on a particular research question
Critical appraisal is meticulous,
typically involving the use of
data extraction forms
A qualitative summary is provided,
quantitative when the data can be
pooled
Usually evidence-based
© 2006-7
Systematic reviews
typically only include RCTs
• Studies are included primarily based on
their quality
– Most systematic reviews only include RCTs
because it is the only design that adequately
controls for confounding variables and biases
– The potential for studies to overrate the
treatment effect is higher when bias is present
• Studies using other less rigorous designs
are usually eliminated
Evidence-based Chiropractic
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Conducting systematic reviews
Evidence-based Chiropractic
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Search strategy
• Should be described in enough detail so
that another researcher could replicate the
results, including:
– Database(s) searched
– Date the search was performed
– Time-frame encompassed by the search
– A list of search terms used
– Languages
Evidence-based Chiropractic
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Search strategy (cont.)
• Conference proceedings, unpublished
studies, and hand-searching of journals
are sometimes included in the search
– If so, the procedures involved should be
described
– Authors must justify using unpublished work
Evidence-based Chiropractic
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Weighting of studies
• Articles may be rejected in a systematic
review due to their poor quality
• Alternatively, studies are
assigned weights in relation
to their assessed validity
– Studies that are more valid will have more
influence on the review’s final results
– Based on methodological quality, width of the
confidence intervals, and external validity
Evidence-based Chiropractic
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Publication bias
• Studies with statistically significant results
are more likely to get published than those
with non-significant results
• Causes of publication bias:
– The author or funding source does not
consider a “failed study” worthy of submission
– Journals are less likely to publish studies that
fail to show positive results
Evidence-based Chiropractic
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Publication bias (cont.)
• Reviews affected by this bias tend to give
an overoptimistic view of the effectiveness
of the therapy
– The chance of this bias occurring is reduced
when authors of systematic reviews search
sources other than journals
• Publication bias in situ
– A type of bias where a portion of a study’s
results are suppressed
Evidence-based Chiropractic
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Meta-analysis
• a.k.a., quantitative systematic review
• A type of systematic review that
statistically combines the results from a
number of studies
• Capable of producing a single estimate of
the effect of a treatment
– Represents the “average” treatment effect
– An estimate of the true treatment effect size
Evidence-based Chiropractic
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Meta-analysis (cont.)
• The same explicit methods as systematic
reviews are utilized
• Systematic reviews and meta-analyses
are are at the top of the hierarchy of
evidence because of their strict
methodology
Evidence-based Chiropractic
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Weighted average
• Meta-analyses typically produce a
weighted average for the treatment effect
estimate
– Small samples are more susceptible to
chance variations than larger studies
– Thus, they are given less weight than larger
studies so they will have less influence on the
final estimate
Evidence-based Chiropractic
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Weighting (cont.)
• Weighting is also based on study quality
– The quality of the individual studies is rated
and resulting numeric scores are calculated
– A corresponding weight is assigned for each
study prior to analysis
Evidence-based Chiropractic
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Meta-analyses can
increase power
• Data from individual studies are combined,
which in effect increases sample size
– Chiropractic studies commonly involve too
few subjects to detect true differences
between the groups
– Pooling data reduces the potential for type II
error
– More likely to detect a treatment effect, if
there actually is one
Evidence-based Chiropractic
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Homogeneity and
heterogeneity
• Homogeneity
– Similarities of included studies that allow them
to be compared
– Homogeneity is preferred in meta-analyses
– Achieved by using suitable inclusion criteria
• Heterogeneity
– Dissimilarities of studies that hamper or even
prevent a realistic comparison of studies
Evidence-based Chiropractic
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Factors that contribute
to heterogeneity
• Heterogeneity in the study samples
– Caused by conflicting inclusion and exclusion
criteria, differences in patients’ baseline health
status, dissimilar geographical locations of
groups, etc.
• Heterogeneity in the study design
– E.g., the way dropouts were managed in the
statistical analysis or the length of time
allowed for patient follow-up
Evidence-based Chiropractic
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Factors that contribute
to heterogeneity (cont.)
• The way patients were handled
– Regarding comorbid conditions, handling of
complications, the control practitioners had in
patient care, or the outcome measures used
• Statistical heterogeneity
– When the observed treatment effects of
studies are more dissimilar than what would
be expected by chance
Evidence-based Chiropractic
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Consequences of
heterogeneity
• When the results of studies in a metaanalysis are inconsistent, it reduces
confidence in its conclusions
• The meta-analysis may actually be
worthless if too dissimilar
– For instance, combining studies that used
different types of comparison groups
– Or outcomes that were dissimilar
Evidence-based Chiropractic
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Forest plot
• A type of graph often used in metaanalyses to illustrate the treatment effect
sizes of the studies
• Each study is represented by a black
square that is an estimate of their effect
sizes
• A horizontal line extends to either side of
the squares, the 95% confidence interval
Evidence-based Chiropractic
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Forest plot
Evidence-based Chiropractic
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Interpreting a forest plot
• If a study’s 95% CI crosses over the
vertical line, it is not statistically significant
• A diamond with a CI line is sometimes
presented at the bottom of the forest plot
to represent an overall estimate
• The black squares may vary in size
representing the weights of the studies
Evidence-based Chiropractic
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Weighting and overall effect
Evidence-based Chiropractic
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Effect size
• The difference between the means of the
treatment and control groups
• When studies are combined in a metaanalysis, the units of measurement are not
always comparable
• Effect sizes are standardized to resolve
this problem producing the standardized
mean difference
Evidence-based Chiropractic
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Standardized mean difference
• The effect size divided by the pooled
standard deviation
– Pooled standard deviation has been adjusted
for the differences in the sizes of the groups
• Represents the standardized difference
between group means
– i.e., the relative magnitude of the
experimental treatment
Evidence-based Chiropractic
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Cohen’s d
Evidence-based Chiropractic
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Odds ratio (OR)
• Cohen’s d is appropriate with continuous
data
• An OR is appropriate when the study’s
outcome measure is dichotomous
– e.g., pain versus no pain
• OR is a comparison of the odds of the
outcome being present in the treatment
group against the control group
Evidence-based Chiropractic
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Relative risk (RR)
• a.k.a., risk ratio
• A comparison of the risk of having the
outcome in the treatment group with that
of the control group
• Sometimes OR is reported and sometimes
RR
– Experts do not agree on which is most
appropriate
Evidence-based Chiropractic
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OR vs. RR
• Consider a hypothetical lower back pain
study with 25 patients in each group
• 5 in treatment group and 10 in control
group are still in pain at the study’s end
5/20
.25
OR =
=
10/15
.66
= .38
5/25
.2
RR =
=
10/25
.4
= .5
Evidence-based Chiropractic
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Meta-analyses are most
valid with RCTs
• However, about half of meta-analyses
include observational studies
– Primarily cohort and case-control
• Observational studies are much more
susceptible to biases and confounding
than RCTs
– Therefore, it is usually inappropriate to
statistically combine the results of such
studies
Evidence-based Chiropractic
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Subgroup analysis
• Meta-analyses typically include patients
with a variety of characteristics
– e.g., age, gender, condition severity, patient
history, etc.
• Patients in these subgroups may respond
to treatment differently
– e.g., low back pain patients with leg pain may
respond to treatment differently than low back
pain only patients
Evidence-based Chiropractic
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Subgroup analysis (cont.)
• Carried out to identify variation between
patient groups regarding certain outcomes
or findings
• The process helps readers to distinguish
the effects of a treatment between
subgroups
• The statistical power of the subgroups’ will
decline as a result
Evidence-based Chiropractic
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Meta-regression
• A statistical procedure that adjusts for
differences between studies in metaanalyses
– May be used in subgroup analyses
• Similar to simple regression
– Predictor variables: characteristics of the
studies
– Outcome variable: treatment effect estimate
Evidence-based Chiropractic
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Sensitivity analysis
• A type of subgroup analysis that considers
non-patient characteristics, e.g., treatment
variations or study methodology
• Determines the extent heterogeneity
affected the results of a meta-analysis
– If the results are weak, sensitivity analysis
may reveal significant treatment effects when
different methods are used
Evidence-based Chiropractic
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Narrative versus systematic
reviews and meta-analyses
• There are no strict rules regarding the
creation of either type
– Therefore, it may be difficult to decide if a
given review is systematic or narrative
• Narrative reviews do not typically use
systematic methods
– They tend to be subjective and prone to bias
– Cover broader topics than systematic reviews
Evidence-based Chiropractic
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Narrative vs.
systematic reviews (cont.)
• May be unclear how conclusions were
drawn from the data in narrative reviews
– Often the number of studies supporting one
side of a topic is counted and then compared
with the number supporting the opposite side
– The side with the highest number of
supporting articles wins
– This process does not consider the weight of
studies as in systematic reviews
Evidence-based Chiropractic
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Advantages and disadvantages of
narrative versus systematic reviews
Narrative
reviews
Advantages
Disadvantages
Present a general overview
covering a specific topic
which provides primary
information and/or an update
Fairly easy for novice
authors to prepare
May not provide the best
available answers to focused
questions
Findings are less reliable
Systematic Present a comprehensive
reviews
review of the literature based
on all available research with
regard to a focused research
question
Provide an estimate of the
“true” answer to the research
question
Evidence-based Chiropractic
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Specialized expertise of
reviewers is required
Involve a formal research
protocol
Findings are only relevant
to a single question
© 2006-7
QUOROM Statement
• QUOROM (Quality of Reporting of Metaanalyses)
• The QUOROM Statement was developed
to reduce the potential for reviews to reach
contradictory conclusions
• An attempt to ensure uniformity and
accurate reporting
• Has been adopted by many journals
Evidence-based Chiropractic
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Structure of review articles
• Should conform to the anatomy of a typical
scholarly article
– i.e., Abstract, Introduction, Methods, Results,
Discussion, and References
• Literature reviews are in reality a type of
research
– However, conclusions are derived from
original sources of information
Evidence-based Chiropractic
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Structured Abstract
• Objective
– The author should clearly state the purpose of
the article
• Background
– A description of what prompted the review
– Presentation of a context for the review
• Methods
– A description of the methods used
Evidence-based Chiropractic
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Structured Abstract (cont.)
• Discussion
– The implication and relevance of the
information the review presents
• Conclusion
– Summary of what the review contributes to
the literature
– What new conclusion can be drawn as a
result of the synthesis of the literature
Evidence-based Chiropractic
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Introduction
• Presents the background and context of
the problem that inspired review
• The topic should be clearly defined, and
tied in with clinical practice
• Novel terms should be defined
• A description of the course of the disease,
common outcomes and treatment options
• A synopsis of existing research
Evidence-based Chiropractic
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Introduction (cont.)
• The importance and need for the review
should be established by showing
– That there are gaps in the literature on the
topic
– The extent of the condition’s negative impact
on society in terms of human suffering and
monetary costs
– Explained in enough detail to substantiate the
need for the review
Evidence-based Chiropractic
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Introduction (cont.)
• A focused and well-constructed question
should be present
– Provides direction for the review
– Assists readers in determining if the review is
applicable to their individual clinical
circumstances
– Should help establish the review’s inclusion
criteria
Evidence-based Chiropractic
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Methods
• Describes the search process and
strategies involved, including:
– Databases searched
– Search terms
– Search limits
• e.g., publication years, languages, ages, etc.
• Should include enough detail to enable
others to replicate the search
Evidence-based Chiropractic
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Methods should include
• The criteria that were used to include or
exclude studies
– For example, exclude surgery related studies
or drug trials
• A description of how studies were
appraised
– Rating instruments are typically used
– However, the reliability of these instruments
varies considerably
Evidence-based Chiropractic
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Methods should include (cont.)
• Information about
– How the relevance of primary studies was
ascertained
– How the data were extracted and synthesized
– Sources of heterogeneity
• How much
• How it was handled
Evidence-based Chiropractic
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Results
• The outcome of the search process is
presented
• Including information on
– The number of articles retrieved
– How many articles were excluded from the
review and which of the inclusion criteria they
failed to meet
• Look for evidence of selective referencing
Evidence-based Chiropractic
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Results (cont.)
• The Results section is sometimes very
short
– When short, details about the retrieved
articles are provided in the Discussion section
• However, it may be longer
– The characteristics of the included studies
may be described and contrasted in this
section
– Often presented in tables
Evidence-based Chiropractic
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Discussion
• The findings of all of the articles in the
review are synthesized to generate a
conclusion
– There may or may not be a separate
Conclusions section
• Information about the etiology,
pathophysiology, diagnosis, treatment, and
prognosis of the condition at issue is often
provided
Evidence-based Chiropractic
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Discussion (cont.)
• Presents a new perspective on the topic
that is usually more reliable than any of
the individual articles in the review
• Caution – authors have the leeway to
defend articles that support their viewpoint
and challenge those that do not
– Systematic methods control for much of this
subjectivity, but it is still possible in the best
types of reviews
Evidence-based Chiropractic
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Discussion (cont.)
• The synthesis is the heart of the literature
review design
– Consequently it is important to ensure that a
meaningful integration is accomplished
• The author should offer an interpretation of
the literature reviewed
– A critical appraisal of the articles reviewed
may be in order
Evidence-based Chiropractic
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Discussion (cont.)
• The conclusion should be in agreement
with the evidence presented in the review
• Authors should emphasize what new
information can be gained
• The conclusion should not merely repeat
what was previously written
Evidence-based Chiropractic
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References
• Should be comprehensive and cite all
articles included in the review
• Derived almost entirely from peerreviewed journals
– But may include conference proceedings,
textbooks, and government documents
– Unpublished works too; but keep in mind, they
have not been peer-reviewed
Evidence-based Chiropractic
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References (cont.)
• No padding of references
– Only enough articles should be cited to make
a point
• Be alert for authors who misrepresent
sources, which is a fairly common
occurrence
– e.g., misquote, selectively quote, omit
relevant information, etc.
– May have to read source material to detect
Evidence-based Chiropractic
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Appraisal of review articles
• Recent reviews are typically more useful
because they contain the latest research
– However, even the latest review may be
behind the most current research
• The literature search should involve
multiple databases
• The search strategy should be clearly
described, along with the criteria used for
study inclusion
Evidence-based Chiropractic
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Appraisal of reviews (cont.)
• Was a clear study question asked?
– Reviews should have a clear purpose in the
form of a study question or statement of
purpose
• Especially with systematic reviews, but also
applies to narrative reviews
– Otherwise, the review will lack direction and
may generate faulty conclusions
Evidence-based Chiropractic
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Appraisal of reviews (cont.)
• Was the study question focused on a
specific clinical issue?
– The study question must be focused on a
specific clinical issue that is relevant to patient
care to help practitioners make clinical
decisions
– Broad topic reviews provide background
information, but are not very helpful in
answering clinical questions
Evidence-based Chiropractic
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Appraisal of reviews (cont.)
• Were the inclusion/exclusion criteria
that were used to select articles
provided and were they appropriate?
– Selection criteria should be appropriate for the
clinical question that prompted the search
– Difficult to assess the breadth of the review or
judge the extent of the author’s partiality
without this information
Evidence-based Chiropractic
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Appraisal of reviews (cont.)
• Was an adequate literature search
conducted?
– The search should be capable of finding all
relevant studies
– Appropriate databases and search terms
– Citations should be collected from the
reference sections of the included articles
– How likely is it that relevant studies were
missed during the search?
Evidence-based Chiropractic
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Appraisal of reviews (cont.)
• Was the validity of the studies included in
the review assessed?
– Reviews that include articles without
appraising them are poorer quality and less
reliable
– Authors may examine issues such as the
methods of randomization and blinding,
whether concealment of the allocation to
groups was used, etc.
Evidence-based Chiropractic
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Appraisal of reviews (cont.)
• Were the studies appraised in a
dependable manner?
– Should be reviewed by more than one
reviewer to strengthen confidence in their
findings
– Reliable appraisal instruments should be used
because they yield more consistent results
• Often included in an appendix
Evidence-based Chiropractic
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Appraisal of reviews (cont.)
• Were the included studies randomized
trials?
– Strengthens the usefulness of a literature
review considerably when RCTS are involved
– Reviews that include observational studies
provide little evidence to support treatment
– However, such reviews are helpful with
patient care and qualify as the best evidence
when observational research is all that exists
Evidence-based Chiropractic
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Appraisal of reviews (cont.)
• Were the results of the included studies
similar?
– Heterogeneity of studies may occur due to:
• Clinical factors (e.g., dissimilar participants,
interventions or outcomes)
• Methodological factors (e.g., sample size or
method of randomization)
• Contradictory results just by chance
– Be concerned when discrepancies are large
Evidence-based Chiropractic
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Appraisal of reviews (cont.)
• Do the results of the synthesis logically
flow from the studies that were included?
– Authors have a great deal of latitude in the
way they interpret and judge articles
– Studies should be weighted based on study
size and validity
• Reviews that simply compare the number of
positive findings to the number with negative
findings may be misleading
Evidence-based Chiropractic
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Appraisal of reviews (cont.)
• If the review dealt with therapy, was an
estimate of treatment effect provided?
– This is the overall estimate of treatment effect
size in a meta-analysis
– The width of the associated confidence
interval reveals the precision of the estimate
– A wide confidence interval degrades precision
and lowers the degree of certainty regarding
the review’s findings
Evidence-based Chiropractic
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Appraisal of reviews (cont.)
• Do the results of the review help with the
care of patients?
– In order to effectively use the results of a
review in the care of a specific patient, the
populations should be similar
– The outcomes used in the review should be
clinically important
– Comparable patient and review interventions
• e.g., are manipulation and mobilization equal
Evidence-based Chiropractic
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Appraisal of reviews (cont.)
• Were directions for future research
offered?
– Especially helpful in reviews because authors
assess the sum of all research on a topic and
are in a very good position to determine what
types of studies should be carried out in the
future
Evidence-based Chiropractic
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