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

5. Data Set
Casework from 2000 to 2007
N = 48

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

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

8. 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.

9. Results – Expert Agreement
Significance

10. Results – Judicial Outcome
Significance

11. Results – Case Type
Significance

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

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

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

15. 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

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

17. 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