The use of error rates to predict and characterize the efficacy of presumptive field tests.

From a forensic perspective, a presumptive test, one which indicates the presence or absence of a certain target material such as blood, is an invaluable tool. Among these tests, there are different specificities, sensitivities, and shelf lives. The accuracy of a test is an algebraic combination of the specificity and sensitivity of the test. Each test has limitations as given by its false positive and false negative rates. The aim of this study was to illustrate how the false positive and false negative rates are to be properly determined using a simulation study for the phenolphthalein test. New presumptive tests must be properly evaluated/validated through testing of commonly encountered household items and other potentially probative items usually found at crime scenes, however, the makeup of test sets must appropriately capture all error rates. In order to correctly use these results when the test is applied to an unknown sample recovered at a crime scene, the error rates cannot be applied directly to estimate whether or not the sample is actually the analyte of interest. In a validation study, the forensic scientist calculates the false positive rate as the p(Positive Reaction|Blood), whereas at the scene, the crime scene investigator wishes to determine the p(Blood|Positive Reaction). All crime scene investigators need to ensure that the conditional is not transposed when interpreting such results. Furthermore, this work provides a model for the assessment of a multiple test diagnostic system intended for investigators.

Evaluating firearm examiner conclusion variability using cartridge case reproductions.

The forensic science pattern comparison areas, including fingerprints, footwear, and firearms, have been criticized for their subjective nature. While much research has attempted to move these disciplines to more objective methods, examiners are still coming to conclusions based on their own training and experience. To complement this subjectivity, black box studies are necessary to establish the accuracy of these feature-comparison methods. However, when cartridges are fired by a firearm to create cartridge case test sets there may be significant variability within the resulting impressions. This can result in different participants receiving test sets with varying levels of difficulty based on differences in impression quality. Therefore, comparison of accuracy between examiners is not straightforward. To compare accuracy between examiners, a method called double-casting was used to create plastic cartridge case reproductions. Double-casts of twenty-one test sets of master cartridge cases were created and mailed to firearm examiners. The double-casts ensured that all participants were comparing exhibits with the same level of detail. The examiners were tasked with determining if the unknown cartridge case in each set was fired by the same firearm as the three knowns. Automated comparisons were also used to compare the cartridge cases within each set. The results from this study showed that there are differences in examiner conclusions when examining the same evidence. Furthermore, it was shown that automated comparison metrics would benefit examiners as a quality control measure to correct any potential errors and strengthen conclusions.

Three-Dimensional Analysis of Cartridge Case Double-Casts.

Due to the shot-to-shot variability in tool mark reproduction on fired cartridge cases, a method of replication is needed for the creation of training and testing sets. Double-casting is one method that has been used for this application, but the accuracy and variability of this method needs to be characterized. Three firearms were used to fire 25 cartridges each to create the master cartridge cases. The double-casting method consists of creating a silicone mold of the master cartridge case. A plastic resin mix is then poured into the mold to create the double-cast reproduction. Fifteen double-casts of each of the 75 fired cartridge cases were created across different silicone molds to analyze within- and between-mold variability. The master cartridge cases and double-casts were scanned with a confocal microscope (Sensofar® S neox) to create three-dimensional representations of the surfaces. Two similarity metrics were used for the objective comparison of the double-casts to their master cartridge cases: the areal correlation coefficient (ACCFMAX ) and the number of congruent matching cells (CMC). The ACCFMAX and CMC data, along with visual examinations, showed that the double-casting method produces accurate reproductions. Within-mold variability was found to be minimal, and between-mold variability was low. These results illustrate that double-casting can be applied for training and testing purposes.

Determining the number of test fires needed to represent the variability present within firearms of various calibers.

The Association of Firearm and Toolmark Examiners recommends a minimum of two test fires be performed when an unknown firearm is submitted to a laboratory prior to doing a comparison with a cartridge case collected from a crime scene. Limited research has been performed to determine how many test fires are necessary to be representative of the match distribution of a firearm. Various makes and models of firearms comprising five calibers were tested using a hybrid equivalence test to determine how many cartridge cases were required to represent the match distribution of an unknown firearm based on both breech face and firing pin correlation scores from an IBIS® HeritageTM System. The same general trend was observed for each caliber of firearm where the equivalence percentage increased from 10 to 30 cartridge cases. Overall, 15 cartridge cases are sufficient for above an 80% probability of representing the full match distribution for an unknown firearm. To approach full equivalence, 25 cartridge cases are enough because 30 cartridge cases were not found to be significantly higher in equivalence percentage for any caliber of firearm tested.

An evaluation of the discriminating power of an Integrated Ballistics Identification System® Heritage™system with the NIST standard cartridge case (Standard Reference Material 2461).

Through analysis and comparison of firing pin, breech face, and ejector impressions, where appropriate, firearm examiners may connect a cartridge case to a suspect firearm with a certain likelihood in a criminal investigation. When a firearm is not present, an examiner may use the Integrated Ballistics Identification System (IBIS®), an automated search and retrieval system coupled with the National Integrated Ballistics Information Network (NIBIN), a database of images showing the markings on fired cartridge cases and bullets from crime scenes along with test fired firearms. For the purpose of measurement quality control of these IBIS® systems the National Institute of Standards and Technology (NIST) initiated the Standard Reference Material (SRM) 2460/2461 standard bullets and cartridge cases project. The aim of this study was to evaluate the overall performance of the IBIS® system by using NIST standard cartridge cases. By evaluating the resulting correlation scores, error rates, and percent recovery, both the variability between and within examiners when using IBIS®, in addition to any inter- and intra-variability between SRM cartridge cases was observed.

Estimation of changes in breech face and firing pin marks over consecutive discharges and its impact on an IBIS® Heritage™ System.

When a firearm is discharged, the individual marks of the breech face and firing pin are imprinted onto the primer of the cartridge case. These individual marks are reproducible between shots; however, over a large number of consecutive shots, it has been observed that minute changes in these individual marks may occur. Changes in individual marks may affect an examiner in their ability to identify or eliminate and may change the magnitude of a likelihood ratio, depending on the system used by the laboratory, but the effect that these changes have on the Integrated Ballistic Identification System (IBIS®) is largely unknown. If such changes negatively affect the performance of the IBIS® then consequences may result with respect to the correct matching candidate not be returned in the top results further compared by an examiner. Two hundred consecutive test fires performed in a clean environment (indoor shooting range) were collected in sequence from 24 new Ruger® SR9 9mm pistols and entered into an IBIS® Heritage™ System. The full known match data were extracted for each firearm, and δ sets were created that had a specific number of cartridge cases between the two cartridge cases being compared. Receiver operating characteristic (ROC) area under the curve (AUC) values for these δ sets were compared to the full known match sets to determine if any significant changes in performance resulted. Although there were instances of significant differences, these only occurred in less than 25% of comparisons, and overall no decreasing trends in performance were observed.

Determining the number of test fires needed to represent the variability present within 9mm Luger firearms.

Many studies have been performed in recent years in the field of firearm examination with the goal of providing an objective method for comparisons of fired cartridge cases. No published research to support the number of test fires needed to represent the variability present within the impressions left on a cartridge case could be found. When a suspect firearm is submitted to a firearm examiner, typically two to four test fires are performed. The recovered cartridge cases are compared to each other to determine which characteristics from the firearm are reproducing, and then compared to any cartridge cases collected at a crime scene. The aim of this research was to determine the number of test fires examiners should perform when a suspect firearm is submitted to the lab to balance cartridge case acquisition time with performance accuracy. Each firearm in the IBIS® database at West Virginia University® is represented by approximately 100 fired cartridge case entries. Random samples of cartridge cases were taken separately from the breech face match score and firing pin match score lists. This subset was compared to the total match distribution of the firearm using a hybrid equivalence test to determine if the subset of similarity scores were statistically equivalent to the larger distribution of scores. For the sampled distribution to remain above 80% equivalent to the match distribution, a minimum of 15 cartridge cases should be used to model the match distribution, based on IBIS® scores. Thirty cartridge cases is a conservative estimate, allowing one to determine that the location and dispersion of the match and sampling distributions are equivalent with nearly 100% probability.

Forensic discrimination of glass using cathodoluminescence and CIE LAB color coordinates: a feasibility study.

Cathodoluminescence (CL) spectroscopy has been shown to be useful for differentiating typical evidentiary glass samples. CL occurs when a surface is bombarded with an electron beam as in scanning electron microscopy and most of this luminescence is in the visible range. In effect, CL imparts color to colorless evidence and as a result, proven methods of forensic color analysis can be applied. In this work, spectral data dimensions were reduced to three and plotted in the CIE LAB color space. This approach allows for incorporation of uncertainties generated principally by intra-sample variation. NIST glass standards were used for method development and validation while potential case applications were studied with collections of window, consumer, and auto headlamp glasses. Using refractive index as the initial grouping variable, all of the window and consumer glasses were differentiated as were 6 of 10 automobile headlamp glasses. The potential advantages of CL include low cost instrumentation, its non-destructive nature, and ease of operation. The current limitations of CL in this context are the lack of databases and standards and the relatively low resolution of typical CL spectra.