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

Abstract

The evaluation of results of forensic genetic analyses given activity level propositions is an emerging discipline in forensic genetics. Although it is a topic with a long history, it has never been considered to be such a critically important topic for the field, as today. With the increasing sensitivity of analysis techniques, and advances in data interpretation using probabilistic models ('probabilistic genotyping'), there is an increasing demand on forensic biologists to share specialised knowledge to help recipients of expert information address mode and timing of transfer and persistence of traces in court. Scientists thereby have a critical role in the assessment of their findings in the context of the case. This helps the judiciary with activity level inferences in a balanced, robust and transparent way, when based on (1) proper case assessment and interpretation respecting the hierarchy of propositions (supported by, for example, the use of Bayesian networks as graphical models), (2) use of appropriate data to inform probabilities, and (3) reporting guidelines by international bodies. This critical review of current literature shows that with certain prerequisites for training and quality assurance, there is a solid foundation for evidence interpretation when propositions of interest are at the 'activity level'.

Bayesian networks for the interpretation of biological evidence

“…The specific probability associated with each node is typically decided by an expert using data from casework, experimentation, validation work, external sources, and sometimes expertise. A recent review article by Taylor, Kokshoorn, and Biedermann () provides a list of five possibilities available to the forensic scientist when determining probabilities for the nodes of a Bayesian network when looking at biological evidence. From the best option to the least favored option, these include: Performing experiments that mimic case circumstances to assign probabilities; Using values from literature that represents similar properties to the case; Considering a range of values that are reasonable for the probability of interest and examining the sensitivity of the LR; Assigning a value based on the expert's experience or knowledge (preferably supported by similar cases); Do not carry out evaluation if there is a lack of data available to ensure a robust opinion is provided. …”

“…When experts do not agree on a particular probability or probabilities, or if there is a range of possible values, sensitivity analysis is a method that can be used to determine how sensitive the LR is to changes in that particular probability (Aitken et al, ; Aitken & Gammerman, ; Biedermann et al, ; Biedermann & Taroni, ; Taylor, Hicks, & Champod, ; Taylor, Kokshoorn, & Biedermann, ). A range of possible values can be used and if the effect on the LR is significant, then either more research is necessary or the network may not be appropriate.…”

“…There are numerous publications summarized by Taylor, Kokshoorn, and Biedermann (), which demonstrate the importance of forensic experts providing knowledge on activity level propositions when evaluating evidence. This has led to an increased awareness throughout the forensic community and in laboratories implementing these techniques for biological evidence, for example, Forensic Science South Australia (Taylor, Kokshoorn, & Biedermann, ) and The Netherlands Forensic Institute provide this service (Kokshoorn et al, ). In their paper, Taylor, Kokshoorn, and Biedermann () provide anecdotal feedback that such evaluations are useful to the courts.…”

“…There has been a reluctance to use Bayesian networks to interpret biological evidence by forensic scientists (Biedermann et al, ; Taylor, Kokshoorn, & Biedermann, ). This is different to distrust of Bayesian methods more generally, for example, the use of the likelihood ratios to evaluate sub‐source (DNA) evidence.…”

“…The use of Bayesian networks to assist in the evaluation of biological evidence has been slow to gain traction, but interest has increased more recently (Biedermann & Taroni, ; De Wolff, Kal, Berger, & Kokshoorn, ; Taylor, Abarno, Hicks, & Champod, ; Taylor, Kokshoorn, & Biedermann, ). Without using a logical framework, it is common for forensic scientists to consider only how likely the DNA evidence is if it came from a particular individual.…”