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Bitemark overlays: an analysis of effectiveness Pretty, Ian A.


It is not unusual to see dentists testifying in Court. Such professionals assist criminal proceedings by identifying the victims of crime and by analysing bitemarks with the hope of determining the biter. Contemporary legal history is littered with cases where it has been possible to match a bite injury on a victim to the person suspected of the crime. In many of the cases, this type of evidence is often crucial to the successful outcome of the trial. Bitemark evidence has been almost universally accepted in the Courts, but the fundamental validity and scientific basis for its use is frequently challenged. Rapid advances in forensic science, particularly within the field of DNA evidence, have caused concern to the judicial system. Recent rulings, such as those of Daubert and Kumho in the United States, have placed a greater emphasis on the validity and reliability of opinion testimony based upon supposed scientific principles. Judges have stated that witnesses must be able to identify published works that define operational parameters of any tests or procedures that form the basis of scientific conclusions. Such works do not exist within the field of bitemark analysis. As the most commonly employed analytical technique in bite injury assessment, this study defines quantifiable variables for transparent overlays. A series of 10 simulated, postmortem bites were created on pigskin and, with accompanying overlays, assembled into cases. Using two separate studies with four examiner groups the study defined values of intra- and inter-examiner reliability, accuracy, sensitivity, specificity, and error rates for transparent overlays. Methods and statistical treatments from medical decision-making and diagnostic test evaluation were employed. Forced decision models and receiver operating characteristic analyses were utilised. The results determined that transparent overlays were effective in the determination of biters. The sensitivity and specificity values were consistent with those of other dental tests, although due to a paucity of equivalent studies it has been impossible to rate the values within a forensic context. The relatively low values of inter-examiner reliability were thought to reflect the nature of both bitemarks as physical evidence and the variability of examiner thresholds. It was concluded that the weak inter-examiner reliability values explain the divergence of odontologists' opinions regarding bitemark identifications often stated in Court. The positive and negative predictive values suggest that bitemarks may be more effective at excluding individuals than including them. The effect of training and experience was found to have little effect on the application of overlays within this study. The work concludes that further research is required within the field of bitemark analysis so that the results of the current study can be placed into context. This work represents a significant first step in establishing the scientific basis for this aspect of forensic dentistry.

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