Evaluation of forensic genetics findings given activity level propositions: A review

An LR framework incorporating sensitivity analysis to model multiple direct and secondary transfer events on skin surface

Bayesian logistic regression is used to model the probability of DNA recovery following direct and secondary transfer and persistence over a 24 h period between deposition and sample collection. Sub-source level likelihood ratios provided the raw data for activity-level analysis. Probabilities of secondary transfer are typically low, and there are challenges with small data-sets with low numbers of positive observations. However, the persistence of DNA over time can be modelled by a single logistic regression for both direct and secondary transfer, except that the time since deposition must be compensated by an offset value for the latter. This simplifies the analysis. Probabilities are used to inform an activity-level Bayesian Network that takes account of alternative propositions e.g. time of assault and time of social activities. The model is extended in order to take account of multiple contacts between person of interest and 'victim'. Variables taken into account include probabilities of direct and secondary transfer, along with background DNA from unknown individuals. The logistic regression analysis is Bayesian - for each analysis, 4000 separate simulations were carried out. Quantile assignments enable calculation of a plausible range of probabilities and sensitivity analysis is used to describe the corresponding variation of LRs that occur when modelled by the Bayesian network. It is noted that there is need for consistent experimental design, and analysis, to facilitate inter-laboratory comparisons. Appropriate recommendations are made. The open-source program written in R-code ALTRaP (Activity Level, Transfer, Recovery and Persistence) enables analysis of complex multiple transfer propositions that are commonplace in cases-work e.g. between those who cohabit. A number of case examples are provided. ALTRaP can be used to replicate the results and can easily be modified to incorporate different sets of data and variables.

A generalised Bayes' factor formula for evidence evaluation under activity level propositions: Variations around a fibres scenario

Generalised Bayes' factors and associated Bayesian networks are developed for the transfer of extrinsic evidence at the activity level, developments that extend previous work on activity level evaluation. A strategy for the assessment of extrinsic evidence is developed in stages with progressive increases in complexity. The final development is illustrated with an example involving fibres from clothing. This provides a list of factors involved in the consideration of a transfer case with activity level propositions and their roles in the determination of evidential value.

An integrated system for forensic DNA testing of sexual assault cases in the Philippines

In the Philippines, more than 7000 cases of sexual assault are reported annually. DNA technology is a powerful tool in identifying assailants. However, it is not routinely used in sexual assault investigations due to insufficient government support to cover the high cost of DNA testing and the absence of a national system for sample collection, handling, storage, and DNA testing of biological evidence. In itself, the nature of sexual assault samples containing DNA mixtures presents challenges to laboratory methods and interpretation of results. The sample recovered from the victim may only contain trace amounts of the assailant’s DNA, may have degraded due to prolonged storage in ambient conditions which is warm and humid in the tropics, or contaminated with inhibitors, such as in anal swabs. Hence, a closer evaluation of the processes of evidence collection and DNA testing is needed to increase the likelihood of success in generating conclusive results. In this paper, we propose an integrated system for DNA testing of biological samples collected from sexual assault victims considering the limitations of resources and the prevailing warm climate. Recommendations in this work should provide basis for formulating national guidelines for DNA analysis in aid of criminal investigations. The proposed scheme can be adopted by forensic DNA laboratories in the Philippines and in other countries facing similar challenges.

Prevalence of DNA of regular occupants in vehicles

People will deposit, redistribute and remove biological traces when they interact with their environment. Understanding the dynamics of trace DNA is crucial to assess both the optimal sampling strategy to recover traces and the relevance of DNA evidence in the context of a case. This paper addresses the prevalence of DNA of drivers, passengers, and unknown individuals in vehicles. Five vehicles with a regular driver only, and five vehicles with a regular driver and regular passenger have each been sampled at twenty locations. Based on the findings, we propose a sampling strategy for investigative purposes as well as for evaluative purposes when evaluating the findings given scenarios that propose the person-of-interest as either the driver or passenger in a vehicle.

Using case specific experiments to evaluate fingermarks on knives given activity level propositions

Bayesian networks have shown to be a useful tool for the evaluation of forensic findings given activity level propositions. In this paper, we demonstrate how case specific experiments can be used to assign probabilities to the states of the nodes of a Bayesian network for the evaluation of fingermarks given activity level propositions. The transfer, persistence and recovery of fingermarks on knives is studied in experiments where a knife is either used to stab a victim or to cut food, representing the activities that were disputed in the case of the murder of Meredith Kercher. Two Bayesian networks are constructed, exploring the effect of different uses of the experimental data by assigning the probabilities based on the results of the experiments. The evaluation of the findings using the Bayesian networks demonstrates the potential for fingermarks in addressing activity level propositions.

Challenges in Human Skin Microbial Profiling for Forensic Science: A Review

The human microbiome is comprised of the microbes that live on and within an individual, as well as immediately surrounding them. Microbial profiling may have forensic utility in the identification or association of individuals with criminal activities, using microbial signatures derived from a personal microbiome. This review highlights some important aspects of recent studies, many of which have revealed issues involving the effect of contamination of microbial samples from both technical and environmental sources and their impacts on microbiome research and the potential forensic applications of microbial profiling. It is imperative that these challenges be discussed and evaluated within a forensic context to better understand the future directions and potential applications of microbial profiling for human identification. It is necessary that the limitations identified be resolved prior to the adoption of microbial profiling, or, at a minimum, acknowledged by those applying this new approach.

DNA transfer to firearms in alternative realistic handling scenarios

Firearms are the most relevant items of evidence in gun-related crimes, likely bearing various traces facilitating an objective reconstruction of the crime. Trace DNA recovered from firearm surfaces might help to identify individual(s) having handled the firearm and thereby possibly to link the firearm and the corresponding shooter, however, the interpretation of DNA traces on handled items can be challenging and requires a detailed understanding of various factors impacting DNA prevalence, transfer, persistence and recovery. Herein, we aimed at improving our understanding of factors affecting the variability of trace DNA characteristics recovered from firearms handled in gun-related crimes: Skin contact traces were recovered from various outer surfaces of two types of firearms handled in four realistic, casework-relevant handling scenarios and the corresponding trace characteristics (DNA yield, number of contributors, relative profile contribution for known and unknown contributors, LRs) were compared. Trace DNA characteristics differed distinctly between handling conditions, firearm and surface types as well as handling individuals and intraindividual deposits emphasizing the variability and complexity of trace DNA profile composition expected to be recovered from firearms after realistic handling scenarios. The obtained results can provide useful insights for forensic experts evaluating alternative activity level propositions in gun-related crimes.

Interpol review of forensic biology and forensic DNA typing 2016-2019

This review paper covers the forensic-relevant literature in biological sciences from 2016 to 2019 as a part of the 19th Interpol International Forensic Science Managers Symposium. The review papers are also available at the Interpol website at: https://www.interpol.int/content/download/14458/file/Interpol%20Review%20Papers%202019.pdf.

Data driven optimization of sexual assault case processing

In recent years, several forensic laboratories have noted an increase in the number of sexual assault cases submitted for testing, often leading to longer turnaround times. In that context, forensic laboratories may be interested in reviewing their procedures to increase productivity. Here, we present two different strategies that were put in place in our laboratory. First, we changed the way sexual assault evidence kits (SAEK) are processed by implementing an optimized workflow that prioritizes the internal samples (vaginal, anal, and oral). This new procedure allowed for a drastic decrease in turnaround time, while maintaining a similar investigative power.

Secondly, we used data from casework to target cases and samples that were likely to yield biological material from the perpetrator, in an attempt to avoid dedicating time and effort to cases for which there is a very low probability of obtaining foreign DNA evidence. Among other things, we looked at the likelihood of obtaining DNA from the perpetrator when the complainant reported the use of a condom, has showered after the assault or when the complainant has no memory of the assault. Results show that those circumstances do not dramatically decrease the probability of finding DNA from the perpetrator.

Structuring cases into propositions, assumptions, and undisputed case information

In this short publication we address the topic of evaluation of biological results given activity level propositions, particularly when the source of the biological material is not in question. When the issue regards the mechanisms or actions that led to the deposition of the biological material concerned, there is a need for more case information than when the issue pertains to the source of the DNA. It is up to the scientist to structure the provided case information into propositions, assumptions and undisputed case information. In our statements, deciding what goes in the propositions and what will be part of the paragraph dedicated to case information is partly personal. It may however affect the way we think about a case and thereby have consequences for the evaluation of the results as well as for the communication of the information to the recipient. In particular we highlight the importance of considering and communicating all DNA transfer mechanisms that are relevant to the evaluation under each proposition, and how the way the propositions are thought about (and expressed) can assist in this endeavour.

Bayesian networks and dissonant items of evidence: A case study

The assessment of different items of evidence is a challenging process in forensic science, particularly when the relevant elements support different inferential directions. In this study, a model is developed to assess the joint probative value of three different analyses related to some biological material retrieved on an object of interest in a criminal case. The study shows the ability of probabilistic graphical models, say Bayesian networks, to deal with complex situations, those that one expects to face in real cases. The results obtained by the model show the importance of a conflict measure as an indication of inconsistencies in the model itself. A contamination event alleged by the defense is also introduced in the model to explain and solve the conflict. The study aims to give an insight in the application of a probabilistic model to real criminal cases.

Distinct spectrum of microRNA expression in forensically relevant body fluids and probabilistic discriminant approach

MicroRNA is attracting worldwide attention as a new marker for the identification of forensically relevant body fluids. A probabilistic discriminant model was constructed to identify venous blood, saliva, semen, and vaginal secretion, based on microRNA expression assessed via RT-qPCR. We quantified 15 candidate microRNAs in four types of body fluids by RT-qPCR and found that miR-144-3p, miR-451a-5p, miR-888-5p, miR-891a-5p, miR-203a-3p, miR-223-3p and miR-1260b were helpful to discriminate body fluids. Using the relative expression of seven candidate microRNAs in each body fluid, we implemented a partial least squares-discriminant analysis (PLS-DA) as a probabilistic discriminant model and distinguished four types of body fluids. Of 14 testing samples, 13 samples were correctly identified with >90% posterior probability. We also investigated the effects of microRNA expression in skin, semen infertility, and vaginal secretion during different menstrual phases. Semen infertility and menstrual phases did not affect our body fluid identification system. Therefore, the selected microRNAs were effective in identifying the four types of body fluids, indicating that probabilistic evaluation may be practical in forensic casework.

Scenario-Entity Analysis based on an entity-relationship model: Revisiting crime reconstruction

For a crime case, the related physical evidence and information can be termed entities, and there exist different types of relationships between entities. Entity-relationship models connect numerous entities through different relationships, which is useful in crime reconstructions. However, two types of problems may occur that can mislead crime reconstructions in the real world. Specifically, important entities may not be collected and vital relationships may go undiscovered. In this paper, we used an approach based on an entity-relationship model to address these problems. We organized the related entities used to reconstruct crimes according to their physical properties and sorted the relationships between entities through temporal, spatial and logical dimensions. The proposed approach is called 'Scenario-Entity Analysis' (SEA), and it uses several steps for discovering entities and relationships. The SEA also provides a framework for associating events/scenarios with evidence, which is important for crime reconstructions. Using a combination of SEA and Bayesian networks, a three-layered Bayesian network was constructed for uncertainty reasoning. A knife-attack case is then presented to demonstrate the analytical process of SEA.

Secondary DNA transfer by working gloves

With the development of highly sensitive STR profiling methods, combined with sound statistical tools, DNA analysis on the (sub-)source level is hardly ever seriously questioned in court. More often, the exact mode of DNA transfer to the crime scene is questioned. In burglary cases, in particular when gloves are worn, secondary DNA transfer is often discussed as explanation for finding a DNA profile matching the accused because it is well known that gloves can act as a potential vector for indirect DNA transfer. In this study we investigated the shedder status as a possible factor influencing the extent of secondary DNA transfer to a crime scene, with the person committing the crime wearing working gloves. Firstly, the shedder status for 40 participants (20 male, 20 female) was determined, following a previously published procedure. Good shedders (n = 12) were found to deposit a higher amount and quality of DNA onto objects, compared to bad shedders (n = 25). Secondly, participants were paired into four groups (good with good; good with bad; bad with good; bad with bad), each group consisting of five pairs. The first participant (P1) of each pair used working gloves to pack and carry a box to simulate a house move. Two days later, the second participant (P2) of the pair wore the same pair of gloves to simulate a burglary, using a screwdriver as a break-in tool. After taking swabs of the outside and inside of a glove (primary DNA transfer) and the handle of the screwdriver (secondary DNA transfer), full DNA analysis was performed. Our experiments show that good shedders, overall, deposit more DNA than bad shedders, both onto the outside and the inside of the glove, regardless of being P1 or P2. When conducting the experiments with two participants sharing the same shedder status, no significant differences occurred in the number of deposited alleles. In six out of 19 cases a DNA profile matching P1 was found (binary LR>10⁶) on the screwdriver and in all six cases P1 was a good shedder. Our results indicate that the shedder status of an individual affects the extent of DNA transfer. They further confirm the possibility of an innocent person's DNA profile being found on an object they never handled.

Assessment of the transfer, persistence, prevalence and recovery of DNA traces from clothing: An inter-laboratory study on worn upper garments

Among the various items recovered from crime scenes or persons involved in a crime event, clothing items are commonly encountered and submitted for forensic DNA sampling. Depending on the case circumstances and the activity-of-interest, sampling of the garment may concentrate on collecting DNA from the wearer, or from one or more offenders who have allegedly contacted the item and/or wearer. Relative to the targeted DNA, background DNA already residing on the item from previous contacts, or transferred during or after the crime event, may also be collected during sampling and observed in the resultant DNA profile. Given our limited understanding of how, and from where, background DNA is derived on clothing, research on the transfer, persistence, prevalence, and recovery (TPPR) of DNA traces from upper garments was conducted by four laboratories. Samples were collected from several areas of two garments, each worn on separate working or non-working days and individually owned by four individuals from each of the four laboratories, and processed from DNA extraction through to profiling. Questionnaires documented activities relating to the garment prior to and during wearing, and reference profiles were obtained from the wearer and their close associates identified in the questionnaire. Among the 448 profiles generated, variation in the DNA quantity, composition of the profiles, and inclusion/exclusion of the wearer and their close associates was observed among the collaborating laboratories, participants, garments worn on different occasions, and garment areas sampled.

The effectiveness of the UK national DNA database

Since the emergence of forensic DNA profiling and the corollary creation of DNA databases, efforts to maximise the efficiency and utility of DNA technology have intensified. Such efforts are expedient given the imperative that expenditure on DNA should be cost-effective and the benefits demonstrable. The practice of retaining DNA profiles in databases, either obtained from individuals involved in criminal investigations, or retrieved from suspected crime scenes, has spread globally. The UK's National DNA Database (NDNAD), created in 1995, is both one of the longest established, and biggest of such forensic DNA databases internationally. As such, it is instructive to look at whether there is evidence to demonstrate the effectiveness of this DNA database. This paper thus examines efforts to gauge the effectiveness of forensic DNA databases, concluding that while the UK NDNAD may have led directly to convictions in high profile crimes, its broader impact upon public security goals remains elusive.

On DNA transfer: The lack and difficulty of systematic research and how to do it better

Since DNA from touched items and surfaces ("touch DNA") can successfully and reliably be analyzed, the question as to how a particular DNA containing sample came to be from where it was recovered is of increasing forensic interest and expert witnesses in court are increasingly challenged to assess for instance whether an incriminatory DNA sample matching to a suspect could have been transferred to the crime scene in an innocent manner and to guess at the probability of such an occurrence. The latter however will frequently entail expressing a subjective probability i.e. simply making a best guess from experience. There is, to the present date, an extensive and complex body of literature on primary, secondary, tertiary and even higher order DNA transfer, its possibility, plausibility, dependency on an array of variables and factors and vast numbers of permutations thereof. However, from our point of view there is a lack of systematic data on DNA transfer with existing research widely varying in quality and relevance. Our aim was, starting from a comprehensive survey of the status quo and appreciating its increasing importance, to in the first part of our review raise consciousness towards the underestimated and insufficiently accounted for complexity of DNA transfer and thus appendant research of forensic scientists serving as expert witnesses in court but also acting in the role of a journal referee to point them to areas of criticism when reviewing a manuscript on DNA transfer. In the second part, we present propositions how to systematize and integrate future research efforts concerning DNA transfer. Also, we present a searchable database providing an extensive overview of the current state of knowledge on DNA transfer, intended to facilitate the identification of relevant studies adding knowledge to a specific question and thus help forensic experts to base their opinion on a broader, more complete and more reproducible selection of studies.

Sharing data on DNA transfer, persistence, prevalence and recovery: Arguments for harmonization and standardization

Sharing data between forensic scientists on DNA transfer, persistence, prevalence and recovery (TPPR) is crucial to advance the understanding of these issues in the criminal justice community. We present the results of a collaborative exercise on reporting forensic genetics findings given activity level propositions. This exercise outlined differences in the methodology that was applied by the participating laboratories, as well as limitations to the use of published data on DNA TPPR. We demonstrate how publication of experimental results in scientific journals can be further improved to allow for an adequate use of these data. Steps that can be taken to share and use these data for research and casework purposes are outlined, and the prospects for future sharing of data through publicly accessible databases are discussed. This paper also explores potential avenues to proceed with implementation and is intended to fuel the discussion on sharing data pertaining to DNA TPPR issues. It is further suggested that international standardization and harmonization on these topics will benefit the forensic DNA community as it has been achieved in the past with the harmonization of STR typing systems.