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In forensic casework it is common that multiple pieces of evidence are obtained in a single case. For the evaluation of such evidence, one cannot simply assume that they originated from the same source. If this can be disputed, one could decide to report the evidential value of the separate pieces by computing likelihood ratios (LRs) for them separately. However, by doing so, information regarding the conditional dependency structure is lost and the potentially difficult step of combining them is left to the trier of fact. Especially in situations where the pieces of evidence are of the same type, a combined evaluation could be substantially more informative. In such situations, it is not unrealistic that the separate evidential values are not optimally combined by the trier of fact. From a scientific point of view, the optimal way is that the forensic scientist combines the forensic evidence in a report using hypotheses at the 'activity level'. In practice, however, this is not always feasible for every case for reasons of efficiency. In this article, we explore what can be done in cases where the optimal solution is too costly. For a combined evaluation, the prior distribution for the number of distinct sources of the pieces of evidence becomes part of the LR. Generally, prior probabilities are beyond the scope of expertise of the forensic specialist and are left to the trier of fact to decide. We propose a lower bound LRapproach for such situations that is independent of the prior distribution. Using a set of examples, we present how this method can be applied in practice. [ABSTRACT FROM AUTHOR]

Experts providing evidence in legal cases are universally recommended to be transparent, particularly in their reasoning, so that legal practitioners can critically check whether the conclusions are adequately supported by the results. However, when exploring the practical meaning of this recommendation it becomes clear that people have different things in mind. The UK appeal court case R v T painfully exposes the different views. In this article we argue that there can be a trade-off between clarity and transparency, and that in some cases it is impossible for the legal practitioner to be able to follow the expert’s reasoning in full detail because of the level of complexity. All that can be expected in these cases is that the legal practitioner is able to understand the reasoning up to a certain level. We propose that experts should only report the main arguments, but must make this clear and provide further details on request. Reporting guidelines should address the reasoning in more detail. Legal practitioners and scientists should not be telling each other what to do in the setting of a legal case, but in other settings more discussion will be beneficial to both. We see the likelihood ratio framework and Bayesian networks as tools to promote transparency and logic. Finally, we argue that transparency requires making clear whether a conclusion is a consensus and reporting diverging opinions on request. [ABSTRACT FROM AUTHOR]

The introduction of DNA evidence has transformed human individualization in criminal litigation, but it also introduced daunting statistical, philosophical and practical problems into the process. The current practice in many legal cases is that a forensic expert reports a match probability or a likelihood ratio. However, the value of the likelihood ratio depends on the particular hypotheses used by the expert. Often there are various choices possible for the hypotheses used, and the corresponding likelihood ratios for different hypotheses are typically different. We therefore argue that the findings of an expert should not be given with a single number, and that any report with a match probability or likelihood ratio should be accompanied by a discussion of the effect of these numbers. We suggest a way to do this, using the so‐called posterior odds, which are invariant under various sets of hypotheses. Our approach is applicable in legal systems where the judge or jury is not allowed to know that the suspect was in a database. However, juridical problems may arise when courts insist on, for example, only a frequency estimate to be reported. [ABSTRACT FROM PUBLISHER]

The article discusses the likelihood-ratio tests and bayesian approach for interpretation of evidences in legal practice. It highlights the differences in results concluded through putting the two logical approaches in determination of forensic evidences. It also refers to the differences between the legal tradition for the requirements of logical inference concluded through bayesian analysis and likelihood-ratio tests for evaluation of evidence by juries.