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Applied forensic epidemiology: a Bayesian evaluation of forensic evidence in a vehicular homicide investigation
Umeå University, Faculty of Medicine, Department of Community Medicine and Rehabilitation, Forensic Medicine.
2009 (English)In: Journal of Forensic and Legal Medicine, ISSN 1752-928X, Vol. 16, no 2, 83-92 p.Article in journal (Refereed) Published
Abstract [en]

The comparative weighting of evidence in a criminal case can be a complicated task when the relevance or meaning of the evidence is disputed. An example of this complexity in seen in vehicular homicide investigations in which the identity of the driver (and thus the guilty party) is not clear. The discipline of Forensic Epidemiology, including the appropriate application of Bayes' Theorem (Bayes' Law) provides a systematic framework to bring clarity to the evaluation of such matters. Bayes' is a useful tool for the conditioning and quantification of probabilities associated with evidence in a vehicular homicide investigation. The authors present a case study in the application of Bayes' Theorem to the facts in a vehicular homicide investigation. An initial analysis of the crash dynamics in comparison with the injury pattern and ejection status of the surviving occupant versus that of the decedent suggested that the survivor was the driver. The results of the analysis were used as tests for guilt, with estimated true and false positive rates, which then formed the basis for a Bayesian calculation of the posterior probability of the survivor's guilt given the evidence. As a result of the Bayesian analysis described herein, it was determined that the survivor was 19 times more likely to have been the driver, in comparison with the decedent. This ratio far exceeded the suggested threshold of 10:1 for establishing the guilt of the survivor beyond a reasonable doubt. When used properly, Bayes' Theorem can offer definitive insight in the investigation and prosecution of vehicular homicide cases.

Place, publisher, year, edition, pages
2009. Vol. 16, no 2, 83-92 p.
National Category
Forensic Science
Research subject
Forensic Medicine
URN: urn:nbn:se:umu:diva-81454DOI: 10.1016/j.jflm.2008.08.017PubMedID: 19135003OAI: diva2:655335
Available from: 2013-10-11 Created: 2013-10-11 Last updated: 2013-10-18Bibliographically approved
In thesis
1. The role of forensic epidemiology in evidence-based forensic medical practice
Open this publication in new window or tab >>The role of forensic epidemiology in evidence-based forensic medical practice
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Objectives This thesis is based on 4 papers that were all written with the same intent, which was to describe and demonstrate how epidemiologic concepts and data can serve as a basis for improved validity of probabilistic conclusions in forensic medicine (FM). Conclusions based on probability are common in FM, and the validity of probabilistic conclusions is dependant on their foundation, which is often no more than personal experience. Forensic epidemiology (FE) describes the use and application of epidemiologic methods and data to questions encountered in the practice of FM, as a means of providing an evidence-based foundation, and thus increased validity, for certain types of opinions. The 4 papers comprising this thesis describe 4 unique applications of FE that have the common goal of assessing probabilities associated with evidence gathered during the course of the investigation of traumatic injury and death.


Materials and Methods Paper I used a case study of a fatal traffic crash in which the seat position of the surviving occupant was uncertain as an example for describing a probabilistic approach to the investigation of occupant position in a fatal crash. The methods involved the matching of the occupants’ injuries to the vehicular and crash evidence in order to assess the probability that the surviving occupant was either the driver or passenger of the vehicle at the time of the crash.

In the second and third papers, epidemiologic data pertaining to traffic crash-related injuries from the National Automotive Sampling System-Crashworthiness Data System (NASS-CDS) was used to assess the utility and strength of evidence, such as vehicle deformation and occupant injury of a particular severity and pattern, as a means of assessing the probability of an uncertain issue of interest. The issue of interest in Paper II was the seat position of the occupant at the time of a rollover crash (similar to Paper I), and the association that was investigated was the relationship between the degree of downward roof deformation and likelihood of a serious head and neck injury in the occupant. The analysis was directed at the circumstance in which a vehicle has sustained roof deformation on one side but not the other, and only one of the occupants has sustained a serious head or neck injury. In Paper III the issue of interest was whether an occupant was using a seat belt prior to being ejected from a passenger vehicle, when there was evidence that the seat belt could have unlatched during a crash, and thus it was uncertain whether the occupant was restrained and then ejected after the seat belt unlatched, or unrestrained. Of particular interest was the relative frequency of injury to the upper extremity closest to the side window (the outboard upper extremity [OUE]), as several prior authors have postulated that during ejection when the seat belt has become unlatched the retracting seat belt would invariably cinch around the OUE and cause serious injury.

In Paper IV the focus of the analysis was the predictability of the distribution of skull and cervical spine fractures associated with fatal falls as a function of the fall circumstances. Swedish autopsy data were used as the source material for this study.

Results In Paper I the indifferent pre-crash probability that the survivor was the driver (0.5) was modified by the evidence to arrive at a post-test odds of 19 to 1 that he was driving.

In Paper II NASS-CDS data for 960 (unweighted) occupants of rollover crashes were included in the analysis. The association between downward roof deformation and head and neck injury severity (as represented by a composite numerical value [HNISS] ranging from 1 to 75) was as follows: for each unit increase of the HNISS there were increased odds of 4% that the occupant was exposed to >8 cm of roof crush versus <8 cm; 6% for >15 cm compared to <8 cm, and 11% for >30 cm of roof crush compared to <8 cm.

In Paper III NASS-CDS data for 232,931 (weighted) ejected occupants were included in the analysis, with 497 coded as seat belt failures, and 232,434 coded as unbelted. Of the 7 injury types included in the analysis, only OUE and serious head injury were found to have a significant adjusted association with seat belt failure, (OR=3.87, [95% CI 1.2, 13.0] and 3.1, [95% CI 1.0, 9.7], respectively). The results were used to construct a table of post-test probabilities that combined the derived sensitivity and (1 - specificity) rates with a range of pre-crash seat belt use rates so that the results could be used in an investigation of a suspected case of belt latch failure.

In Paper IV, the circumstances of 1,008 fatal falls were grouped in 3 categories of increasing fall height; falls occurring at ground level, falls from a height of <3 meters or down stairs, and falls from ≥3 meters. Logistic regression modeling revealed significantly increased odds of skull base and lower cervical fracture in the middle (<3 m) and upper (≥3 m) fall height groups, relative to ground level falls, as follows: (lower cervical <3 m falls, OR = 2.55 [1.32, 4.92]; lower cervical ≥3 m falls, OR = 2.23 [0.98, 5.08]; skull base <3 m falls, OR = 1.82 [1.32, 2.50]; skull base ≥3 m falls, OR = 2.30 [1.55, 3.40]). Additionally, C0-C1 dislocations were strongly related to fall height, with an OR of 8.3 for the injury in a ≥3 m fall versus ground level.

Conclusions In this thesis 4 applications of FE methodology were described. In all of the applications epidemiologic data resulting from prior FM investigations were analyzed in order to draw probabilistic conclusions that could be reliably applied to the circumstances of a specific investigation. It is hoped that this thesis will serve to demonstrate the utility of FE in enhancing evidence-based practice in FM.


Place, publisher, year, edition, pages
Umeå: Umeå universitet, 2013. 64 p.
Umeå University medical dissertations, ISSN 0346-6612 ; 1598
forensic epidemiology, forensic medicine, probability, traffic crash, autopsy, logistic regression, Bayes´law
National Category
Forensic Science
Research subject
Forensic Medicine; Forensic Medicine
urn:nbn:se:umu:diva-81434 (URN)978-91-7459-729-5 (ISBN)
Public defence
2013-11-06, Hörsal B, Norrlands universitetssjukhus, 901 85 UMEÅ, 09:00 (English)
Available from: 2013-10-16 Created: 2013-10-10 Last updated: 2013-10-16Bibliographically approved

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