False negative probability in iGSR detection: a Bayesian approach

Abstract

This article discusses the detection of inorganic gunshot residue (iGSR) particles through scanning electron microscopy with energy dispersive X-ray spectrometer (SEM/EDS) in the discovery step. We calculated the probability that all characteristic inorganic gunshot residue particles (iGSR) go undetected and their dependence on the image pixel resolution setup. We built and validated a iGSR particle detection model that relates particle size with equipment registers, and we applied it to 1,174 samples analyzed by a forensic science laboratory. Our results indicate that the probability of missing all characteristic iGSR particles is below 5% for pixel sizes below $0.32 \mu m^{2}$. These values indicate that pixel sizes as great as the double in area that is commonly used in laboratory casework, $0.16 \mu m^{2}$, are effective for an initial scanning of a sample as it yields good rates of detection of characteristic particles, which might exponentially reduce laboratory workload.