A Mixture Model for the Number of Gunshot Residues Found on Suspects' Hands

Quantifying the strength of gunshot residue (GSR) evidence requires scientific knowledge about the number of particles expected to be found on individuals who were or were not involved in a shooting. However, controlled experiments demand expensive resources in terms of microscope time and labor, wh...

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Bibliographic Details
Published in:Journal of forensic sciences 2020-07, Vol.65 (4), p.1114-1119
Main Authors: Benzaquen, Daniel, Mandel, Micha, Israelsohn Azulay, Osnat, Zidon, Yigal, Cohen, Yaron
Format: Article
Language:English
Subjects:
Algorithms
Antimony - analysis
Barium - analysis
Firearm discharge residue
Forensic Ballistics - methods
Forensic science
GSR
gunshot residue
Hand
Humans
iterative algorithm
Iterative algorithms
Iterative methods
Lead - analysis
likelihood ratio
Microscopy, Electron, Scanning
mixture model
Models, Statistical
negative binomial
poison distribution
Police
Residues
Shootings
Skin - chemistry
Spectrometry, X-Ray Emission
Wounds, Gunshot
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Summary:Quantifying the strength of gunshot residue (GSR) evidence requires scientific knowledge about the number of particles expected to be found on individuals who were or were not involved in a shooting. However, controlled experiments demand expensive resources in terms of microscope time and labor, which restricts the data of most studies to only a small group of individuals. We suggest a novel method that exploits data collected routinely on suspects during the daily work of forensic laboratories. These observational data relate to both persons who were involved in a shooting and innocent individuals. We suggest a mixture approach with different models for the number of gunshot residue particles in each group and develop an iterative algorithm to estimate the probabilities of observing the evidence under the defense proposition that the suspect is innocent and under the prosecution assumption that he is not. The method is applied to data of more than 500 suspects collected by the Israel National Police Division of Identification and Forensic Science. The analysis shows that the probability of finding three or more GSR particles on the hands of innocent suspects is very small, less than 1.5 in 1000 cases. Our new method enables researchers to use data on real cases, possibly supplemented by experimental data, in order to estimate the probabilities of a given GSR finding under the defense and prosecution propositions.
ISSN:0022-1198
1556-4029
DOI:10.1111/1556-4029.14310