Current fire debris analysis techniques do not include a method of determining the strength of the evidential result obtained from the analysis, nor does it include a method of determining the error rates inherent to the analytical process. The current standard method of analyzing fire debris evidence is ASTM-E1618-19, which allows the inclusion of qualifying statements to augment the interpretation of the results that are required to be reported in categorical terms instead of in terms indicating evidentiary strength. A new workflow based in linear sequential unmasking within the guidelines of the ASTM-E1618-19 protocol was applied by each of three different analysts individually to 20 known ground truth data samples after analyzing them via gas chromatography-mass spectrometry. Each individual analyst’s optimal threshold for determining the presence of ignitable liquids was then determined by applying decision theory to the values assigned to the samples by the analysts using the same scoring system contained in the workflow. Casework relevant samples for which the ground truth was unknown were then analyzed using the same method. Receiver operating characteristics curves and assigned scores were used to report the three analysts’ validated performances. The results suggest that this new approach to forensic analysis is a possible method for validating analyst performance in forensic laboratories based on evidential strength and error rates and may help limit the need for qualifying statements included in fire debris analysis.

Forensic workflow using linear sequential unmasking and decision theory

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ACS Fall 2022

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