A survey of the distribution of glass on clothing

Estimating probability terms for the background presence of glass when considering activity in forensic casework

Coulson et al. [1] proposed methodology for the estimation of the P and S terms used in glass interpretation when assessing the value of the findings given activity level propositions. These terms arise in a model proposed by Evett [2], Evett and Buckleton [3], and are based on survey data. Specifically they proposed a model for estimating Pk, k = 0, 1, 2, … and Sn, n = 1, 2, where Pk is the probability of finding k distinct sources (or groups) of glass on a person of interest (POI), and Sn is the probability that the kth source consists of n fragments. In this article we make a number of extensions to the work of Coulson et al. [1]. Firstly we derive an estimate of the uncertainty in the parameter of the Coulson et al. model, and show how this may be used-for example, to compute an estimate of how the probabilities may vary or how to compare estimates resulting from different surveys. Secondly, we extend the model by allowing a more sensible modelling of the "excess" zeros (in the case of the P terms) and excess ones (in the case of the S terms). The methodology used to make these extensions relies on purely frequentist theory of estimation in keeping with the original work. A Bayesian approach to estimation will be the subject of future work. Additionally, we demonstrate the use of an R (R Core Team, [4]) package, called fitPS (Curran, [5]) which makes the methodology described in this article easy to implement in practice.

Freshwater diatom persistence on clothing II: Further analysis of species assemblage dynamics over investigative timescales

Diatoms are a useful form of environmental trace evidence, yielding a circumstantial link between persons and scenes of forensic interest. A developing empirical research base has sought to understand those factors affecting the transfer and persistence of freshwater diatoms on clothing and footwear surfaces. Although an initial study has demonstrated that diatoms can persist on clothing following weeks of wear, no previous research has explored the temporal dynamics of a persistent species assemblage over timescales pertinent to forensic investigations. This study therefore aimed to determine if: (1) valve morphology (size and shape) influences diatom persistence, (2) the relative abundance of taxa within an assemblage affects retention, and (3) a persistent diatom assemblage retrieved from clothing after one month can reliably be compared to the site of initial transfer. To build on previous research findings which highlighted the impact of substrate and environmental seasonality on diatom transfer and persistence, here, nine clothing materials were tested in spring before a seasonal comparison in the winter. Fabric swatches were immersed in a freshwater river, worn attached to clothing, and subsamples retrieved at regular intervals (hours, days, weeks) up to one month post-immersion. Diatoms were extracted using a H2O2 technique and analysed via microscopy. The results indicated that smaller diatoms (< 10 µm) are retained in significantly greater abundance, with no statistically significant difference between centric and pennate diatom loss over time. Although a persistent species assemblage was relatively stable over the one month of wear, significant differences were identified between clothing substrate in the spring and between the seasonal samples. The most abundant environmental taxa were consistently identified in the forensic samples, with greater variability attributed to the retention of relatively less common species. The findings suggest that, despite a loss in the abundance and species-richness of diatoms retrieved from clothing over time, a persistent assemblage may provide a useful circumstantial link to the site of initial transfer. The complex relationships between clothing type, environmental seasonality, and time since wear on retention, emphasise the need for diatom trace evidence to be carefully interpreted within an exclusionary framework, and the significance of any casework findings to be determined with reference to empirical evidence bases.

Evaluation of glass evidence at activity level: A new distribution for the background population

For evidence evaluation of the physicochemical properties of glass at activity level a well-known formula introduced by Evett & Buckleton [1,2] is commonly used. Parameters in this formula are, amongst others, the probability in a background population to find on somebody's clothing the observed number of glass sources and the probability in a background population to find on somebody's clothing a group of fragments with the same size as the observed matching group. Recently, for efficiency reasons, the Netherlands Forensic Institute changed its methodology to measure not all the glass fragments but a subset of glass fragments found on clothing. Due to the measurement of subsets, it is difficult to get accurate estimates for these parameters in this formula. We offer a solution to this problem. The heart of the solution consists of relaxing the assumption of conditional independence of group sizes of background fragments, and modelling the probability of an allocation of background fragments into groups given a total number of background fragments by a two-parameter Chinese restaurant process (CRP) [3]. Under the assumption of random sampling of fragments to be measured from recovered fragments in the laboratory, parameter values for the Chinese restaurant process may be estimated from a relatively small dataset of glass in other relevant cases. We demonstrate this for a dataset of glass fragments collected from upper garments in casework, show model fit and provide a prototypical calculation of an LR at activity level accompanied with a parameter sensitivity analysis for reasonable ranges of the CRP parameter values. Considering that other laboratories may want to measure subsets as well, we believe this is an important alternative approach to the evaluation of numerical LRs for glass analyses at activity level.

A survey of glass found on the headwear and head hair of a random population vs. people working with glass

This study investigated the prevalence of glass particles on the headwear and head hair of two different population groups; the general public who do not work with glass, and glaziers from O'Brien(®) Glass Industries who work with glass and have regular contact with broken glass. The 232 samples collected from the head hair and headwear from the random population resulted in the recovery of 6 glass fragments in total on 6 individuals (i.e. one fragment each). All of these fragments were from head hair samples with no multiple fragments recovered. The two headwear samples that were taken revealed no fragments. These results were in contrast to the survey that was conducted on the head hair and headwear of 25 glaziers from O'Brien(®), in which 138 glass fragments were found in total on 24 of the 25 glaziers. The size and number of fragments found in each sample were also generally larger for the glaziers group. The results from this study indicate that the prevalence of glass on the head hair and head wear of the random population is very low in comparison to the head hair and headwear of those who have regular contact with breaking glass. The significance of this finding with respect to the interpretation of glass evidence is also discussed.

A model for estimating the number of glass fragments transferred when breaking a pane: experiments with firearms and hammer

Numerous publications have shown the importance of transfer in the interpretation of glass evidence. As this phenomenon is also highly variable, it was decided to test the hypothesis that there exists a means to predict the number of fragments recovered at time t = 0. Panes of float glass-of different types and thickness-were broken using either a firearm or a hammer. It was decided to choose a firearm as the main breaking device, as it allowed not only to have more reproducible conditions but also to acquire knowledge in a field where little has been published. Despite the inherent variation in the breaking process, the results show that using a statistical model it is possible to predict the number of fragments transferred onto a garment from the number of fragments transferred to the ground. This research also indicates the size and number of particles transferred onto a person, when breaking window panes of different types (float, laminated or toughened) with different breaking procedures.

Glass particles in footwear of members of the public in south-eastern Australia--a survey

A total of 776 pairs of shoes collected from random members of the public in south-eastern Australia were examined for the presence of glass fragments. From the samples collected a total of 110 fragments were recovered from 57 pairs of shoes (7.3% of the pairs examined). This study shows that the prevalence of glass fragments in footwear is dependent upon the area of the shoe from which the fragments were recovered. A much higher percentage of shoes were found to have fragments embedded in the sole (5.9%) than in the upper area of the shoe (1.9%). These shoes were also more likely to have multiple fragments from multiple sources of glass. Only a very small percentage of shoes contained fragments in both the upper and the sole (0.3%). These findings and their significance for the interpretation of glass evidence involving footwear are discussed in this study.

Glass on clothing and shoes of members of the general population and people suspected of breaking crimes

The outer clothing and footwear of 122 people attending a university gymnasium and a private gymnasium were searched for fragments of glass. Both the surfaces and the pockets of the clothing and the uppers and soles of the footwear were searched. New Zealand forensic glass cases have been reviewed to determine the amount of non-matching glass present on the clothing of people who are suspected of breaking crimes. Data from 114 suspects who had no matching glass on their clothing and shoes were accumulated. Statistical modelling techniques have been applied to the data collected.

More on the hierarchy of propositions: exploring the distinction between explanations and propositions

Interpretation of the weight of scientific evidence depends upon the framing of at least two competing propositions to weigh against each other. It is the stage of framing propositions that is the most difficult aspect of evidence interpretation. The logical structure for case assessment and interpretation has been described by the authors in three previous papers [Cook R, et al. A model for case assessment and interpretation. Science & Justice 1998; 38: 151-156. Cook R, et al. A hierarchy of propositions: deciding which level to address in casework. Science & Justice 1998; 38: 231-239. Cook R, et al. Case pre-assessment and review in a two-way transfer case. Science & Justice 1999; 39: 103-111]. This paper considers the framing of propositions in greater detail, in particular the intermediate stage of exploring less formal explanations. All of the discussion is based on experiences encountered in workshops with caseworking forensic scientists.

A model for case assessment and interpretation

The authors describe a new approach to decision-making in an operational forensic science organization based on a model, embodying the principles of Bayesian inference, which has been developed through workshops run within the Forensic Science Service for forensic science practitioners. Issues which arise from the idea of pre-assessment of cases are explored by means of a case example.

Sampling in forensic comparison problems

When someone breaks glass a number of tiny fragments may be transferred to that person. If the glass is broken in the commission of a crime then these fragments may be used as evidence. If a large number of fragments are recovered from the suspect, then it may be more efficient for the forensic scientist to examine a subset of these fragments. Such sampling incurs information loss. This paper will derive an expression that allows a partial quantification of this loss. The loss of such information due to the examination of a subset of recovered material arises with many forms of evidence.

The acquisition of breaking and broken glass

A set of experiments tested the idea that an examiner can distinguish glass acquired by backward fragmentation from that acquired by contact with already-broken glass. A window was broken. The exterior surface of the breaker's clothing was brushed and hair-combings taken and these were searched for glass fragments. Wearing the same clothing the breaker then cleared up the broken glass and further hair-combings and brushings of the clothing surface were taken. The size, shape and presence or absence of an original flat surface was recorded for the recovered fragments. The distributions within these parameters were compared for the two methods of acquisition and association between the method and the parameters was tested using contingency tables. It was found that fragment shape does not identify the method of acquisition and that a high proportion of fragments retaining the original outer surface shows that the glass was acquired by backward fragmentation.

Bayesian framework for the evaluation of fibre transfer evidence

The application of the likelihood ratio, derived in a Bayesian framework, to different case scenarios involving fibre evidence has permitted the authors to identify and evaluate the dominant parameters and their effect on the likelihood ratio. Moreover, it has been emphasized that these parameters are not only defined by the conditions of the contact, but also by the strategy chosen by the defence.

DNA evidence: wrong answers or wrong questions?

Much of the controversy over DNA evidence is due to the way in which forensic scientific evidence has classically been presented. The orthodox approach is to consider whether two samples match according to a predetermined criterion. If they do, the fact of match is reported along with an estimate of the frequency of the characteristics. This method fails to address the questions raised in court cases, diverts argument into irrelevancies and stultifies research. Presentation of evidence in the form of likelihood ratios, on the other hand, forces the witness to answer the questions the court is interested in and makes apparent lines of research required to increase our understanding.