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A Brief Guide to Critical Literature Review
Cathy Hollister, RDH, MSPH, PhD
Nashville Area Dental Support Center
Session Goal
To review key concepts in interpreting current, relevant
dental research so that clinicians can use
appropriate publications for clinical decision making.
Learning Objectives
At the end of this session, participants will be able
 Name a strength and weakness of review articles
and original research reports
 Explain the benefits of a quasi-experimental
study design
 Explain the importance of internal and external
 Interpret a p value
Key Points to Consider:
Peer Reviewed Publications
1. Is the material primary or secondary?
2. What was the study design?
3. Internal Validity: does the study measure want was
4. External Validity: can the results be generalized
5. Statistics
Are the results statistically significant?
2. Are the results clinically significant?
What is the Publication Type?
Standard Review
Primary Research
 Strengths
 Includes a full description of research
 Focused
 Controls for confounding variables (the ability to
control for other variables differs by study design)
 Weaknesses
 Scope is limited
 May not be generalizable to other populations or times
Review Articles
 Strengths
 Includes relevant material from many types of studies
 Presents studies conducted over a period of time
 Weaknesses
 The reader may be unable to evaluate
appropriateness of the articles included in the review
 May present only one point of view
Review Article
 Consider the review article: Mercury Toxicity and
Treatment: A review of the literature
 Notice the lack of strict criteria that opens the
possibility of author bias to stress a particular point
of view
 Notice also that for the reader, it can be very difficult
to evaluate the quality of the reviewed articles
 Overall conclusion: Mercury is toxic
Systematic Reviews
 A specific type of review article that has strict
inclusion criteria resulting in:
 Only high quality research is included
 Selection bias is reduced
Systematic Review
 The Cochrane Collaboration conducts systematic
reviews on a variety of topics.
 Weakness
 Few studies meet inclusion criteria, therefore it can be
difficult to draw strong conclusions
Example: Dental Amalgam and Multiple Sclerosis: A
Systematic Review and Meta-Analysis
 Overall Conclusion:
Insufficient evidence, need more study 
Original Research
 Now consider Neurobehavioral Effects of Dental
Amalgam in Children
 This Randomized Clinical Trial measured the impact
of mercury exposure in dental amalgam on
neurobehavioral assessments.
 Notice the narrow focus of the research and the
specific means of measuring the impact of mercury
 Overall conclusion: Dental amalgam poses no
significant neurobehavioral risk in children over
the age of 7 in Portugal 
3 Articles: Different Conclusions
 These articles had a common topic: dental amalgam
and the possible consequences to exposure to
 These were all published in peer reviewed journals
 Consider the similarities and differences in the
 What would you consider to be a strength and
weakness of each article?
 What overall conclusions could you draw after reading
these 3 publications?
Primary Research
Key Points to Consider
 Study Design
 Validity
 Internal
 External
 Statistics
What is the Study Design?
Clinical Trials
Ethnography or
Case Control
Cross Sectional
Single or
multiple case
Time or Case
 Strengths
 Determines Causality
 Risk of other factors is minimized
 Determines dose response
 Weaknesses
 Expensive
 May be unethical
 May have small sample sizes
 May not replicate real life situations
Quasi-Experimental Design
 Cohort
 A group with similar characteristics followed through
 Case Control
 Identify people with a condition (cases) and very similar
people without the condition (controls)
 Compare previous exposures
 Time Series
 Multiple cross sectional surveys
Quasi-Experimental Design
 Strengths
 Less expensive
 Avoids ethical concerns
 More likely to replicate real situations
 Weaknesses
 Usually includes biases
 Many variables not under strict control
 Confounding variables may not be eliminated
Confounders: Crime & Ice Cream
 Crime increases in the summer
 Ice cream consumption increases in the summer
Eating ice cream causes crime
Criminals like ice cream
Article Review: Maternal Amalgam
 Study design, Validity, Statistical significance, clinical
 Are the conclusions are supported by the data?
 Potential sources of bias?
 Are there confounders?
 What can you learn from this study?
 What questions ARE NOT answered in this study?
Study Design
 Descriptive, observational
 Retrospective (to determine previous exposures)
 Strengths
 Reflects real life situation
 Inexpensive and no ethical concerns
 Weaknesses
 Cannot determine causality
 Bias and confounders
Validity: Internal and External
 Internal
 Is the study free from bias?
 Did the study measure what was intended?
 External
 Can you generalize the results to other groups?
Internal Validity
 Did the study
measure what was
 Even with the best
study design, sources
of bias may be
unavoidable and may
affect study’s impact
Common Threats to Internal
 Selection Bias: some participants were
systematically excluded from the study
 Measurement error: study does not measure what
was intended to be measured
 Recall Bias: people do not remember past events
 Ambiguity about the direction of the causal
relationship: Which came first, chicken or egg? 
External Validity
 How generalizable are the results of the study?
 Even with excellent internal validity, the results may
not be applicable to your population of interest due to
systematic differences.
 Example:
Race, gender, and socioeconomic status are common risk
factors for many diseases. Results of a periodontal study on
healthy adults may not apply to adults with diabetes.
 Statistics are based on probability.
 Some natural variation will always occur within
 Statistics are used to test the likelihood that findings
are the result of the intervention and not a result of
this natural variation.
 Statistics are used to project if similar findings would
occur in any other sample or in the overall
P Value
 A p value is a measure of the likelihood that the
results of the study happened BECAUSE of the
intervention, and not because of normal variations in
the study group.
 The smaller the p value, the more significant the
finding. 
 A report of p<.05 means that ,“There is less than a
5% probability that the study findings happened by
chance and chance alone.”
 p<.01 means, “There is less that 1% probability that
the findings are due to chance and chance alone.”
Clinical vs Statistical Significance
 If the results of the study reach statistical
significance, consider if the finding is really important
clinically 
 Example:
 A periodontal intervention reduced pocket depth by
0.03mm (p<.01) (Statistically significant, or the result
was due to the intervention and not a result of normal
variation among the study participants)
 Is a gain of 0.03mm important to periodontal health?
(Clinically significant)
 Every scientific publication has weaknesses, no
clinical question can be answered by a single study
or article
 Repeated results lend strength to conclusions
 Consider the differences between the study
population and YOUR population
 Statistical significance may not mean clinical