Classification of firing pin impressions using HOG-SVM.

Crimes, such as robbery and murder, often involve firearms. In order to assist with the investigation into the crime, firearm examiners are asked to determine whether cartridge cases found at a crime scene had been fired from a suspect's firearm. This examination is based on a comparison of the marks left on the surfaces of cartridge cases. Firing pin impressions can be one of the most commonly used of these marks. In this study, a total of nine Ruger model 10/22 semiautomatic rifles were used. Fifty cartridges were fired from each rifle. The cartridge cases were collected, and each firing pin impression was then cast and photographed using a comparison microscope. In this paper, we will describe how one may use a computer vision algorithm, the Histogram of Orientated Gradient (HOG), and a machine learning method, Support Vector Machines (SVMs), to classify images of firing pin impressions. Our method achieved a reasonably high accuracy at 93%. This can be used to associate a firearm with a cartridge case recovered from a scene. We also compared our method with other feature extraction algorithms. The comparison results showed that the HOG-SVM method had the highest performance in this classification task.

Establishing likelihood ratios for evaluating opposing propositions concerning the activity causing methamphetamine contamination: Smoking or manufacture?

In New Zealand, concerns have been raised over the presence of methamphetamine contamination in households, especially when the activity causing the contamination is unknown. The cause of contamination is also a contentious issue in clandestine laboratory cases concerning charges in relation to "Use of Premises" (Section 12: Misuse of Drugs Act 1975, New Zealand). Regardless of the cause, other than scientific opinion, there is currently no analytical technique that can satisfactorily address the provenance of methamphetamine residues. For several years, approximate methamphetamine contamination levels have been collected from suspected clandestine laboratories in New Zealand, where methamphetamine is believed to have been manufactured. This study used this data and compared it to similar data from properties where the drug is suspected to have been used (smoked) to model likelihood ratios (LR). It is well documented that the LR forms the backbone to a Bayesian method of interpreting forensic evidence. As such, this data has the potential to underpin a novel Bayesian approach in the evaluation of clandestine laboratory evidence.

Persistence of Polydimethylsiloxane Condom Lubricants.

Polydimethylsiloxane (PDMS) is commonly used to lubricate condoms. The detection of PDMS on swabs from complainants can be used to support an allegation of sexual assault. Previous research has focused on establishing analytical techniques for detecting PDMS. This research examined the persistence of PDMS on the penis, in the vagina, in the mouth, and on skin. The longest PDMS detection times were 20 h on the penis, 35 h in the vagina, and 52 h on skin. PDMS was detected up to 4 h in the mouth if the participant did not eat or drink and up to 9 h if the participant slept. PDMS was not detected in the mouth after eating or drinking. The presence of biological fluids had no detrimental effect on the analysis. Aqueous extraction of swabs for DNA did not remove any significant amount of PDMS; hence, swab remains could be subsequently analyzed for PDMS.

Application of likelihood ratios for firearm and toolmark analysis.

Historically firearm and toolmark examiners have rendered categorical or inconclusive opinions and eschewed probabilistic ones, especially in the United States. We suggest this practice may no longer be necessary or desirable, and outline an alternative approach that is within a comprehensive logical/Bayesian paradigm. Hypothetical forensic and non-forensic examples are provided for readers who are practicing firearm and toolmark examiners, and the strengths and weaknesses of both approaches are considered.

Experimental design for acquiring relevant data to address the issue of comparing consecutively manufactured tools and firearms.

The comparison of consecutively manufactured tools and firearms has provided much, but not all, of the basis for the profession of firearm and toolmark examination. The authors accept the fundamental soundness of this approach but appeal to the experimental community to close two minor gaps in the experimental procedure. We suggest that "blinding" and attention to appropriateness of other experimental conditions that would consolidate the foundations of our profession. We do not suggest that previous work is unsound.