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Expert Disagreement in Bitemark Casework

Expert Disagreement in Bitemark Casework . C. Michael Bowers DDS, JD, SCSA (IAI) Iain Pretty BDS(Hons), M.Sc. PhD., MFDS RCS (Ed). AMERICAN ACADEMY OF FORENSIC SCIENCES. PRESENTED FEBRUARY 2008 – WASHINGTON, DC. Purpose for Case Research.

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Expert Disagreement in Bitemark Casework

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  1. Expert Disagreement in Bitemark Casework C. Michael Bowers DDS, JD, SCSA (IAI) Iain Pretty BDS(Hons), M.Sc. PhD., MFDS RCS (Ed) AMERICAN ACADEMY OF FORENSIC SCIENCES PRESENTED FEBRUARY 2008 – WASHINGTON, DC

  2. Purpose for Case Research Produce quantitative score values for bitemark cases using the BM Severity and Significance Scale (BMSSS). Determine the forensic identification quality of bitemark evidence admitted in US courts. Determine Error Rates of cases using DNA

  3. Method Development and Validation of a Human Bitemark and Severity Scale. J Forensic Sci, May 2007, Vol.52, No. 3 Iain A. Pretty

  4. Data Set • Casework from 2000 to 2007 • N = 48

  5. Case Details • Range of descriptors for each case • E.g. Post conviction DNA exoneration • E.g. Disagreement between pretrial dentists

  6. Methods • For each case category a weighted mean BMSSS value calculated • Undertaken blinded to the category of case

  7. Results • Due to large number of categories only summary results shown here • A total of 37 categories were assessed • In brief, there appears to be an association between bitemarks scored between 1 – 2 and post conviction exonerations.

  8. Results – Expert Agreement Significance

  9. Results – Judicial Outcome Significance

  10. Results – Case Type Significance

  11. Discussion • BMSSS is not a linear scale • Suggests trends in bitemark severity and significance • Significant differences between scores and case categories

  12. Discussion • Cases in which a conviction was secured had a similar score to those in which DNA exoneration was the final outcome (1.8)

  13. Discussion • Child abuse cases have significantly lower BMSSS scores than other cases • Reasons for this? • Closed population? • Desire to help by prosecution dentist?

  14. Discussion • 25 litigated cases • 6 resulted in a DNA exoneration • Indicates a potential error rate of 24% • 68% cases featured significant differences of opinion between forensic scientists of similar experience and training • Unheard of in other disciplines

  15. Limitations • Sample may not be representative • Random • Independently assessed for BMSSS score • More cases may strengthen the trends seen

  16. Conclusions • Initial look at trends between bitemark significance and case characteristics • Conviction and exoneration cases are similar – (significance value = 1 – 2. The evidence generally have only class characteristics. • Opinions of experts show no agreement between prosecution and the defense

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