Assessing the Strength of Trace Evidence Fracture Fits through a Comprehensive, Systematic and Quantifiable Approach
Tatiana Trejos, Aldo Romero
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Abstract
Criminal activities such as sexual assaults, kidnappings, hit and runs, and homicides often lead to the fracturing of materials. The realignment between fragments left at the scene and those items recovered from an individual, or object of interest, could become crucial evidence during the investigation. These fracture fits are often regarded as the highest degree of association of trace materials due to the common belief that fracture edges often produce individualizing patterns. Nonetheless, there is a need to demonstrate the scientific validity of this assumption. Currently, the examination of fracture edges involves the subjective judgment of the examiner without quantifiable uncertainty. There are no consensus-based standard methodologies for the identification of distinctive features on a fractured edge, a systematic criterion for informing a fit/non-fit decision, or methods for assessing the weight of the evidence. Thus, there is a critical need to develop, validate, and standardize fracture fit examinations and their respective interpretation protocols. In the absence of such foundations, the assessment of the value of the evidence and the reliability of the expert’s testimony would remain challenging.
The overall goal of this research was to develop an effective and practical approach that provides an empirically demonstrable basis to assess the significance of trace evidence fracture fits. Our specific goals were first, to develop a systematic method for the comparison of fracture fits of common trace materials such as duct tapes, textiles, and automotive plastics, using both human-based protocols and automated computational algorithms. Second, to develop a relevant extensive database of nearly 9,000 samples to evaluate performance rates in this field, and assess the weight of a fracture fit using similarity metrics, probabilistic estimates, and score likelihood ratios. Third, to evaluate the utility and reliability of the proposed approach through inter-laboratory studies that can establish consistency base rates. The partnership of experienced forensic researchers, computational material science physicists, statisticians, and practitioners was crucial for developing strategies to facilitate the future adoption of the developed approaches within crime laboratories.
This research specifically addressed several research needs identified by NIST-OSAC and the NIJ- TWG (quantitative assessment of error rates, scientific foundations, standardization, validation, interpretation, casework review, and proficiency assessment). As a result, this study is anticipated to transform current trace evidence practice by providing—for the first time—harmonized examination protocols and decision thresholds, effective mechanisms to ensure transparent and systematic peer- review process and interlaboratory testing, and quantitative basis that substantiate the evidential value
of fracture fit conclusions.