1. Reading the Dental Literature
A Brief Guide to Critical Literature Review
Cathy Hollister, RDH, MSPH, PhD
Nashville Area Dental Support Center

2. Session Goal
To review key concepts in interpreting current, relevant dental research so that clinicians can use appropriate publications for clinical decision making.

3. Learning Objectives
At the end of this session, participants will be able to:
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 validity
Interpret a p value

4. Key Points to Consider: Peer Reviewed Publications
Is the material primary or secondary?
What was the study design?
Internal Validity: does the study measure want was intended
External Validity: can the results be generalized
Statistics
Are the results statistically significant?
Are the results clinically significant?

5. What is the Publication Type?
Research
(Primary)
Experiments
Quasi-Experimental
(quantitative)
Descriptive
Qualitative
Reviews
(Secondary)
Standard Review
Systematic Review
Commentary
Editorial
Opinion/Position

6. Primary Research Strengths Weaknesses
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

7. Review Articles Strengths Weaknesses
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

8. 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 

9. 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

10. Insufficient evidence, need more study 
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 

11. 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 exposure.
Overall conclusion: Dental amalgam poses no significant neurobehavioral risk in children over the age of 7 in Portugal 

12. 3 Articles: Different Conclusions
These articles had a common topic: dental amalgam and the possible consequences to exposure to mercury
These were all published in peer reviewed journals
Consider the similarities and differences in the conclusions.
What would you consider to be a strength and weakness of each article?
What overall conclusions could you draw after reading these 3 publications?

13. Primary Research Key Points to Consider Study Design Validity
Internal
External
Statistics

14. What is the Study Design?
Experiments
Randomized Clinical Trials
Pretest-Intervention-Posttest
Quasi-Experimental
Cohort
Case Control
Time or Case Series
Descriptive
Epidemiological Surveys
Surveillance
Cross Sectional
Qualitative
Ethnography or Phenomenology
Grounded Theory
Single or multiple case studies

15. Experiments:RCT Strengths Weaknesses 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

16. Quasi-Experimental Design
Cohort
A group with similar characteristics followed through time
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

17. 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

18. Confounders: Crime & Ice Cream
Crime increases in the summer
Ice cream consumption increases in the summer
Therefore:
Eating ice cream causes crime OR
Criminals like ice cream

20. Article Review: Maternal Amalgam 
Study design, Validity, Statistical significance, clinical significance
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?

21. Study Design  Strengths Descriptive, observational
Retrospective (to determine previous exposures) 
Strengths
Reflects real life situation
Inexpensive and no ethical concerns
Weaknesses
Cannot determine causality
Bias and confounders

22. Validity: Internal and External
Is the study free from bias?
Did the study measure what was intended?
External
Can you generalize the results to other groups?

23. Internal Validity Did the study measure what was intended?
Even with the best study design, sources of bias may be unavoidable and may affect study’s impact
Bias
Causal Relationships
Confounders

24. Common Threats to Internal Validity
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 accurately
Ambiguity about the direction of the causal relationship: Which came first, chicken or egg? 

25. 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.

26. Probability Statistics are based on probability.
Some natural variation will always occur within groups.
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 population.

27. 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.”

28. 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)

29. Summary
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 significance

30. Questions?