Enhancing the forensic comparison process of common trace materials through the development of practical and systematic methods

An ongoing advancement in forensic trace evidence has driven the development of new and objective methods for comparing various materials. While many standard guides have been published for use in trace laboratories, different areas require a more comprehensive understanding of error rates and an urgent need for harmonizing methods of examination and interpretation. Two critical areas are the forensic examination of physical fits and the comparison of spectral data, which depend highly on the examiner’s judgment. The long-term goal of this study is to advance and modernize the comparative process of physical fit examinations and spectral interpretation. This goal is fulfilled through several avenues: 1) improvement of quantitative-based methods for various trace materials, 2) scrutiny of the methods through interlaboratory exercises, and 3) addressing fundamental aspects of the discipline using large experimental datasets, computational algorithms, and statistical analysis. A substantial new body of knowledge has been established by analyzing population sets of nearly 4,000 items representative of casework evidence. First, this research identifies material-specific relevant features for duct tapes and automotive polymers. Then, this study develops reporting templates to facilitate thorough and systematic documentation of an analyst’s decision-making process and minimize risks of bias. It also establishes criteria for utilizing a quantitative edge similarity score (ESS) for tapes and automotive polymers that yield relatively high accuracy (85% to 100%) and, notably, no false positives. Finally, the practicality and performance of the ESS method for duct tape physical fits are evaluated by forensic practitioners through two interlaboratory exercises. Across these studies, accuracy using the ESS method ranges between 95-99%, and again no false positives are reported. The practitioners’ feedback demonstrates the method’s potential to assist in training and improve peer verifications. This research also develops and trains computational algorithms to support analysts making decisions on sample comparisons. The automated algorithms in this research show the potential to provide objective and probabilistic support for determining a physical fit and demonstrate comparative accuracy to the analyst. Furthermore, additional models are developed to extract feature edge information from the systematic comparison templates of tapes and textiles to provide insight into the relative importance of each comparison feature. A decision tree model is developed to assist physical fit examinations of duct tapes and textiles and demonstrates comparative performance to the trained analysts. The computational tools also evaluate the suitability of partial sample comparisons that simulate situations where portions of the item are lost or damaged. Finally, an objective approach to interpreting complex spectral data is presented. A comparison metric consisting of spectral angle contrast ratios (SCAR) is used as a model to assess more than 94 different-source and 20 same-source electrical tape backings. The SCAR metric results in a discrimination power of 96% and demonstrates the capacity to capture information on the variability between different-source samples and the variability within same-source samples. Application of the random-forest model allows for the automatic detection of primary differences between samples. The developed threshold could assist analysts with making decisions on the spectral comparison of chemically similar samples. This research provides the forensic science community with novel approaches to comparing materials commonly seen in forensic laboratories. The outcomes of this study are anticipated to offer forensic practitioners new and accessible tools for incorporation into current workflows to facilitate systematic and objective analysis and interpretation of forensic materials and support analysts’ opinions

Assessing the reliability of physical end matching and chemical comparison of pressure sensitive tapes

Pressure sensitive tapes are a common evidence type in a variety of forensic cases, including violent crimes, drug trafficking, and terrorism. Forensic laboratories are often requested to analyze tape samples for composition and potential source identification. Tape samples manufactured from different sources are often distinguishable when using a combination of sensitive analytical techniques. Nonetheless, the interpretation of the data remains challenging due to a lack of standard criteria and guidelines from which examiners can support their conclusions. As a result, there is a need to standardize the protocols of interpretation of physical and chemical comparisons of tape evidence. The manufacture and composition of duct tapes and electrical tapes contribute to physical and chemical properties that require different approaches for source attribution. Duct tape physical features are highly variable and can be used to distinguish between different samples. For samples with consistent physical characteristics, the distinctive realignment of their edges indicates the samples were once part of the same piece. However, regardless of the evidential value, there are no defined protocols on how to substantiate the identification of a fracture fit. Conversely, in electrical tapes which have fewer physical features to evaluate, the elemental analysis is often more informative than the physical examination. The long-term goal of this study is to assess the reliability of physical end matching of duct tape and chemical comparison of electrical tape to provide more objective criteria to inform the examiner’s opinion. The first goal of this study was to develop a systematic protocol to quantify the quality of a fit between duct tape edges, assess the performance of the method, and provide a statistical assessment of the weight of the evidence. The quantitative criteria proposed is an edge similarity score (ESS) - the relative number of matching scrim bins across the tear out of the total number of bins along the length of the fracture. The scrim bins provide a consistent and quantitative measure of the quality of a match and means for a systematic peer review process. A set of 2280 duct tape end comparisons (including hand-torn and scissor-cut edges, and stretched and pristine samples of different quality grade) were evaluated to validate the method. The evidentiary value of physical fits was evaluated through similarity metrics, probabilistic estimates from the distribution of ESS, and score-based likelihood ratios (SLR) to offer statistical support for a match/non-match conclusion. No false positives were found for any of the sets. The accuracy for the low and mid-quality sets ranged between 99.5-99.8% with false negative rates of 1-2%. The high-quality set showed higher uncertainty, with a 21.4% false negative rate, and overall accuracy of 84.9%. On average, ESS higher than 80% provided a score-based likelihood ratio (SLR) that supported the conclusion of a match, while ESS lower than 25% provided an SLR supporting the conclusion of non-match. The second objective was to provide enhanced reliability of chemical comparisons of electrical tapes. X-ray fluorescence (XRF) was proposed as an alternative elemental analysis technique for the chemical comparisons of electrical tapes. While elemental analysis by scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) is currently the most informing method for the characterization of electrical tape backings, XRF offered superior sensitivity and enhanced discrimination capabilities. A set of 40 electrical tape backings known to originate from different sources was used to assess discrimination and classification capabilities of the method. The discrimination for this tape set for XRF ranged from 81.5-91.0% depending on the instrumental configuration, whereas SEM-EDS only achieved a discrimination power of 78.8%. XRF also showed to be comparable to the discrimination capability of LA-ICP-MS (84.6% discrimination power) for this set of tapes. In addition, a semi-quantitative method for estimation of significant differences among samples was evaluated to complement spectral overlay. This study developed a quantitative evaluation of the quality of a fracture match and offered an empirical basis that substantiates the reliability of duct tape physical fits and offers statistical support for examiners to inform their opinion. Moreover, it is anticipated that XRF will be easily incorporated in the near future for electrical tape analysis, since it is a technique that is widely applied in crime laboratories for other materials and shown to offer superior performance than conventional methods. The methodologies applied in these studies align with current protocols and instrumentation available in forensic laboratories and lay a foundation for the future development of additional systematic methods for other trace materials