A study of examiner accuracy in cartridge case comparisons. Part 1: Examiner error rates.

This report describes a study undertaken to estimate examiner (not laboratory) error rates for false Identifications and false Eliminations when comparing an unknown to a collection of three known cartridge cases. Volunteer active examiners with Association of Firearm and Toolmark Examiners (AFTE) membership or working in laboratories that participate in the Association of Crime Laboratory Directors (ASCLD) were provided with 15 sets of three known and one questioned cartridge cases fired from a collection of 25 new Ruger SR9 handguns. Remington 9-mm Luger (manufacturer designation L9MM3) ammunition was used and comparison sets were made up of cartridge cases fired within 100 cartridges of each other for each gun. Examiners were provided with a background survey, an answer sheet allowing for the AFTE Range of Conclusions, and return shipping materials. In addition to determining whether the known and questioned cartridge cases were fired with the same handgun, examiners were also asked to assess how many of the three knowns in each set were suitable for comparison, providing an estimated rate of how often each firearm used in the study produces useable, quality marks. The participating examiners were provided with both same-source and different-source comparison sets allowing the study to assess both error rates. Responses were received from 218 participating examiners. The overall rate of false Eliminations was estimated as 0.367% from comparisons known to be from the same firearm but reported as Eliminations. The overall rate of false Identifications was estimated as 1.01% from comparisons known to be from different firearms but reported as Identifications. The rates are not uniform across the sample population with a few examiners providing most of the false Identification responses. Rates of poor-quality mark production varied across the 25 sample handguns; those rates were 2.3% ( ± 1.4%). Both false Elimination and false Identification rates are comparable to or lower than the rate of production of poor-quality marks by the firearms used in this study.

A study of examiner accuracy in cartridge case comparisons. Part 2: Examiner use of the AFTE range of conclusions.

This report describes an analysis of how examiners used the Association of Firearm and Toolmark Examiners (AFTE) Range of Conclusions in a controlled study undertaken to estimate examiner error rates in comparing cartridge cases. Results of the error rate analysis are reported in [1]; this paper focuses on a broader analysis of how the entire collection of classification categories, especially those in the Inconclusive range, were used by the participating examiners. Volunteer active examiners with AFTE membership or working in laboratories that participate in Association of Crime Laboratory Directors (ASCLD) were provided with 15 sets of three known and one questioned cartridge cases fired from a collection of 25 new Ruger SR9 handguns. Remington 9-mm Luger (manufacturer designation L9MM3) ammunition was used and comparison sets were made up of cartridge cases fired within 100 cartridges of each other for each gun. Examiners were provided with a background survey, an answer sheet allowing for the AFTE Range of Conclusions, and return shipping materials. The participating examiners were provided with both same-source and different-source comparison sets allowing the study to assess both error rates. Responses were received from 218 participating examiners. The overall rate of false-negatives was estimated as 0.367 % from comparisons known to be from the same firearm but reported as eliminations. The overall rate of false-positives was estimated as 1.01 % from comparisons known to be from different firearms but reported as identifications. In the case of true different-source examinations, it is clear that the three Inconclusive categories and the Elimination category are not used consistently by all examiners. We identify five different apparent patterns of use of the AFTE Range of Conclusions scale, and discuss possible reasons for and implications of these differences.

Activation of Ras overcomes B-cell tolerance to promote differentiation of autoreactive B cells and production of autoantibodies.

Newly generated immature B cells are selected to enter the peripheral mature B-cell pool only if they do not bind (or bind limited amount of) self-antigen. We previously suggested that this selection relies on basal extracellular signal-regulated kinase (Erk) activation mediated by tonic B-cell antigen receptor (BCR) signaling and that this signal can be replaced by an active rat sarcoma (Ras), which are small GTPase proteins. In this study we compared the activity of Ras and Erk in nonautoreactive and autoreactive immature B cells and investigated whether activation of Ras can break tolerance. Our results demonstrate lower levels of active Erk and Ras in autoreactive immature B cells, although this is evident only when these cells display medium/high avidity for self-antigen. Basal activation of Erk in immature B cells is proportional to surface IgM and dependent on sarcoma family kinases, whereas it is independent of B-cell activating factor, IFN, and Toll-like receptor signaling. Ectopic expression of the constitutively active mutant Ras form N-RasD12 in autoreactive cells raises active Erk, halts receptor editing via PI3 kinase, and promotes differentiation via Erk, breaking central tolerance. Moreover, when B cells coexpress autoreactive and nonautoreactive BCRs, N-RasD12 leads also to a break in peripheral tolerance with the production of autoantibodies. Our findings indicate that in immature B cells, basal activation of Ras and Erk are controlled by tonic BCR signaling, and that positive changes in Ras activity can lead to a break in both central and peripheral B-cell tolerance.

The effect of firearm muzzle gases on the backspatter of blood.

Injuries caused by gunshots can produce what bloodstain pattern analysts know as "backspatter." Observations about the presence or absence of backspatter on an individual may be used in court as evidence of guilt or innocence. The discharge of three firearms (.22 caliber revolver, .38 caliber revolver, and .308 caliber rifle) and the resulting impact of bullets on a blood source were recorded using high-speed digital video imaging. Blood droplets, firearm muzzle gases, and ballistic shock waves were visualized using standard reflected light and shadowgraphy imaging techniques. A significant interaction between air currents, muzzle gases, and particulate material emanating from the firearms upon discharge with backspattered blood was observed. Blood droplets, initially spattered back toward the firearm and the shooter, were observed to change direction under the influence of firearm-induced air currents and were blown forward toward and beyond their original source location. Implications for experts testifying in court and for bloodstain pattern instructors are discussed.

Analysis of glass fragments by laser ablation-inductively coupled plasma-mass spectrometry and principal component analysis.

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) is used to differentiate glass samples with similar optical and physical properties based on trace elemental composition. Laser ablation increases the number of elements that can be used for differentiation by eliminating problems commonly associated with dissolution and contamination. In this study, standard residential window and tempered glass samples that could not be differentiated by refractive index or density were successfully differentiated by LA-ICP-MS. The primary analysis approach used is Principal Component Analysis (PCA) of the complete mass spectrum. PCA, a multivariate analysis technique, provides rapid analysis of samples without time-consuming pair-wise comparison of calibrated analyses or prior knowledge of the elements present in the samples. Probabilities for positive association of the individual samples are derived from PCA. Utilization of the Q-statistic with PCA allowed us to distinguish all samples within the set to a certainty greater than the 99% confidence interval.

Multivariate pattern matching of trace elements in solids by laser ablation inductively coupled plasma-mass spectrometry: source attribution and preliminary diagnosis of fractionation.

Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) is used with two variations of principal components analysis (PCA) for objective, routine comparisons of forensic materials without time-consuming and destructive sample dissolution. The relative concentrations of trace elements in a solid sample are examined to provide a "fingerprint" composition that can be used for identification and source matching of the material. Residue samples are matched to bulk materials using PCA. Variation of laser focus and PCA are also used to diagnose the severity of elemental fractionation in two metal samples that are prone to fractionation, brass and steel. Such fractionation remains the most significant limitation for accurate quantitative analyses by LA-ICPMS.