Date of Graduation
Eberly College of Arts and Sciences
Forensic and Investigative Science
Fracture fits are often regarded as the highest degree of association of trace materials due to the common belief that inherently random fracturing events produce individualizing patterns. Often referred to as physical matches, fracture matches, or physical fits, these assessments consist of the realignment of two or more items with distinctive features and edge morphologies to demonstrate they were once part of the same object. Separated materials may provide a valuable link between items, individuals, or locations in forensic casework in a variety of criminal situations. Physical fit examinations require the use of the examiner’s judgment, which rarely can be supported by a quantifiable uncertainty or vastly reported error rates.
Therefore, there is a need to develop, validate, and standardize fracture fit examination methodology and respective interpretation protocols. This research aimed to develop systematic methods of examination and quantitative measures to assess the significance of trace evidence physical fits. This was facilitated through four main objectives: 1) an in-depth review manuscript consisting of 112 case reports, fractography studies, and quantitative-based studies to provide an organized summary establishing the current physical fit research base, 2) a pilot inter-laboratory study of a systematic, score-based technique previously developed by our research group for evaluation of duct tape physical fit pairs and referred as the Edge Similarity Score (ESS), 3) the initial expansion of ESS methodology into textile materials, and 4) an expanded optimization and evaluation study of X-ray Fluorescence (XRF) Spectroscopy for electrical tape backing analysis, for implementation in an amorphous material of which physical fits may not be feasible due to lack of distinctive features.
Objective 1 was completed through a large-scale literature review and manuscript compilation of 112 fracture fit reports and research studies. Literature was evaluated in three overall categories: case reports, fractography or qualitative-based studies, and quantitative-based studies. In addition, 12 standard operating protocols (SOP) provided by various state and federal-level forensic laboratories were reviewed to provide an assessment of current physical fit practice. A review manuscript was submitted to Forensic Science International and has been accepted for publication. This manuscript provides for the first time, a literature review of physical fits of trace materials and served as the basis for this project.
The pilot inter-laboratory study (Objective 2) consisted of three study kits, each consisting of 7 duct tape comparison pairs with a ground truth of 4 matching pairs (3 of expected M+ qualifier range, 1 of the more difficult M- range) and 3 non-matching pairs (NM). The kits were distributed as a Round Robin study resulting in 16 overall participants and 112 physical fit comparisons. Prior to kit distribution, a consensus on each sample’s ESS was reached between 4 examiners with an agreement criterion of better than ± 10% ESS. Along with the physical comparison pairs, the study iii included a brief, post-study survey allowing the distributors to receive feedback on the participants’ opinions on method ease of use and practicality. No misclassifications were observed across all study kits. The majority (86.6%) of reported ESS scores were within ± 20 ESS compared to consensus values determined before the administration of the test. Accuracy ranged from 88% to 100%, depending on the criteria used for evaluation of the error rates. In addition, on average, 77% of ESS attributed no significant differences from the respective pre-distribution, consensus mean scores when subjected to ANOVA-Dunnett’s analysis using the level of difficulty as blocking variables. These differences were more often observed on sets of higher difficulty (M-, 5 out of 16 participants, or 31%) than on lower difficulty sets (M+ or M-, 3 out of 16 participants, or 19%). Three main observations were derived from the participant results: 1) overall good agreement between ESS reported by examiners was observed, 2) the ESS score represented a good indicator of the quality of the match and rendered low percent of error rates on conclusions 3) those examiners that did not participate in formal method training tended to have ESS falling outside of expected pre-distribution ranges. This interlaboratory study serves as an important precedent, as it represents the largest inter-laboratory study ever reported using a quantitative assessment of physical fits of duct tapes. In addition, the study provides valuable insights to move forward with the standardization of protocols of examination and interpretation.
Objective 3 consisted of a preliminary study on the assessment of 274 total comparisons of stabbed (N=100) and hand-torn (N=174) textile pairs as completed by two examiners. The first 74 comparisons resulted in a high incidence of false exclusions (63%) on textiles prone to distortion, revealing the need to assess suitability prior to physical fit examination of fabrics. For the remaining dataset, five clothing items were subject to fracture of various textile composition and construction. The overall set consisted of 100 comparison pairs, 20 per textile item, 10 each per separation method of stabbed or hand-torn fractured edges, each examined by two analysts. Examiners determined ESS through the analysis of 10 bins of equal divisions of the total fracture edge length. A weighted ESS was also determined with the addition of three optional weighting factors per bin due to the continuation of a pattern, separation characteristics (i.e. damage or protrusions/gaps), or partial pattern fluorescence across the fractured edges. With the addition of a weighted ESS, a rarity ratio was determined as the ratio between the weighted ESS and non-weighted ESS. In addition, the frequency of occurrence of all noted distinctive characteristics leading to the addition of a weighting factor by the examiner was determined. Overall, 93% accuracy was observed for the hand-torn set while 95% accuracy was observed for the stabbed set. Higher misclassification in the hand-torn set was observed in textile items of either 100% polyester composition or jersey knit construction, as higher elasticity led to greater fracture edge distortion. In addition, higher misclassification was observed in the stabbed set for those textiles of no pattern as the stabbed edges led to straight, featureless bins often only associated due to pattern continuation. The results of this study are anticipated to provide valuable knowledge for the future development of protocols for evaluation of relevant features of textile fractures and assessments of the suitability for fracture fit comparisons.
Finally, the XRF methodology optimization and evaluation study (Objective 4) expanded upon our group’s previous discrimination studies by broadening the total sample set of characterized iv tapes and evaluating the use of spectral overlay, spectral contrast angle, and Quadratic Discriminant Analysis (QDA) for the comparison of XRF spectra. The expanded sample set consisted of 114 samples, 94 from different sources, and 20 from the same roll. Twenty sections from the same roll were used to assess intra-roll variability, and for each sample, replicate measurements on different locations of the tape were analyzed (n=3) to assess the intra-sample variability. Inter-source variability was evaluated through 94 rolls of tapes of a variety of labeled brands, manufacturers, and product names. Parameter optimization included a comparison of atmospheric conditions, collection times, and instrumental filters. A study of the effects of adhesive and backing thickness on spectrum collection revealed key implications to the method that required modification to the sample support material Figures of merit assessed included accuracy and discrimination over time, precision, sensitivity, and selectivity. One of the most important contributions of this study is the proposal of alternative objective methods of spectral comparisons. The performance of different methods for comparing and contrasting spectra was evaluated. The optimization of this method was part of an assessment to incorporate XRF to a forensic laboratory protocol for rapid, highly informative elemental analysis of electrical tape backings and to expand examiners’ casework capabilities in the circumstance that a physical fit conclusion is limited due to the amorphous nature of electrical tape backings.
Overall, this work strengthens the fracture fit research base by further developing quantitative methodologies for duct tape and textile materials and initiating widespread distribution of the technique through an inter-laboratory study to begin steps towards laboratory implementation. Additional projects established the current state of forensic physical fit to provide the foundation from which future quantitative work such as the studies presented here must grow and provided highly sensitive techniques of analysis for materials that present limited fracture fit capabilities.
Brooks, Evie K., "Statistical Assessment of the Significance of Fracture Fits in Trace Evidence" (2020). Graduate Theses, Dissertations, and Problem Reports. 7704.