Reading the Dental Literature

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?

6. 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

7. 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

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. 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?

15. Experiments:RCT • 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

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 • 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 • Internal • 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

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?