The impact of chemical trace evidence on justice outcomes: exploring the additive value of forensic science disciplines

The focus of this research was to examine the contribution chemical trace evidence makes to criminal justice outcomes. The aim of this work was to place the discipline of chemical trace evidence under the spotlight as there is a dearth of robust research on the impact of this discipline. In this study, data relating to the forensic examinations in a sample of 238 cases which included chemical trace evidence, was collated with data from police investigations and court processes. The findings show that chemical trace evidence is frequently used in combination with other forensic disciplines to support the progress of high-level criminal cases through the justice system. Due to characteristics of how the criminal cases in the dataset were investigated and prosecuted, in combination with the methodology applied in this study, the impact of forensic evidence on the decision to charge suspects could not be analysed quantitatively. However, the impact of forensic evidence on court outcomes in the sample of cases was analysed using methodology that considered the results of the examinations, and the ability of the evidence to provide support for the inclusion or exclusion of persons of interest. The possibility of chemical trace evidence having impact when applied in combination with other forensic disciplines was also examined. It was found that biological examination results was a significant standalone predictor of court outcomes. In contrast, chemical trace examinations did not predict court outcomes when considered as a standalone predictor but examination results of chemical trace evidence in combination with ballistics/tool marks was significantly associated with court outcomes. The findings of this research indicate that, to assess the full impact of any discipline of forensic evidence on the criminal justice system, the analysis must take into account the potential for important synergies that may exist with other forensic and non-forensic evidence.