Implications of two backward blood spatter models based on fluid dynamics for bloodstain pattern analysis

Using the likelihood ratio in bloodstain pattern analysis

There is an apparent paradox that the likelihood ratio (LR) approach is an appropriate measure of the weight of evidence when forensic findings have to be evaluated in court, while it is typically not used by bloodstain pattern analysis (BPA) experts. This commentary evaluates how the scope and methods of BPA relate to several types of evaluative propositions and methods to which LRs are applicable. As a result of this evaluation, we show how specificities in scope (BPA being about activities rather than source identification), gaps in the underlying science base, and the reliance on a wide range of methods render the use of LRs in BPA more complex than in some other forensic disciplines. Three directions are identified for BPA research and training, which would facilitate and widen the use of LRs: research in the underlying physics; the development of a culture of data sharing; and the development of training material on the required statistical background. An example of how recent fluid dynamics research in BPA can lead to the use of LR is provided. We conclude that an LR framework is fully applicable to BPA, provided methodic efforts and significant developments occur along the three outlined directions.

Inter-observer error for area of origin analysis using FARO Zone 3D

In a bloodletting incident, the area of origin (AO) of an impact bloodstain pattern is crucial when establishing the sequence of events. The use of laser scanners and other three-dimensional (3D) technologies to document and analyse bloodstains have been the subject of previous papers, especially where AO analysis is concerned. FARO Zone 3D (FZ3D) is a relatively new software programme that can be used for bloodstain AO analysis. FZ3D requires a greater understanding of inter-observer errors associated with AO. This study looked at the inter-observer variation between 21 examiners when repeatedly calculating the AO six times for a single impact pattern on a plain white wall. An impact rig which consisted of a spring tension arm was positioned and fixed 45 cm from the X wall (right wall), and 45 cm from the Y wall (left wall). This experimental design resembles an impact blow for a bloodletting event. The AO was unknown to all examiners, making it a blind study. The collected results were documented in a Microsoft Excel datasheet and later analysed. From previous literature, a 30 cm acceptable allowance was utilised for AO analysis; however, there is currently no accepted standard error for this type of analysis. The overall total 3D mean error for all examiners was 5.62 cm. The maximum error for any one impact analysis was 24.27 cm. The variation of the data, which was collected by all examiners, was documented as X = 1.14 cm, Y = 1.24 cm, Z = 1.68 cm, and the total 3D error = 2.28 cm. The total 3D error for any one examiner and the variance between examiners did not exceed the 30 cm acceptable allowance utilised in previous literature.

Bloodstain pattern analysis: Does experience equate to expertise?

Bloodstain pattern analysis (BPA) has long been accepted by courts as an area of expertise; however, that position has recently been challenged. The discipline has been criticized for limited empirical research into practitioner determination error rates and whether determinations require specialized knowledge/expertise, including whether practitioner experience level influences accuracy. This study attempted to address these knowledge gaps as they relate to bloodstain pattern recognition. The aims were twofold: to establish whether practitioners would outperform lay non-practitioners, and whether practitioner experience influenced accuracy and error in determinations. Comparisons of practitioner responses under three scenarios (forced, casework, and definitive) were also made to assess conservatism/certainty in pattern recognition. Participants (both BPA practitioners and non-practitioners) analyzed photographs of bloodstain patterns and made determinations of the broad bloodstain category and specific bloodstain pattern type. When forced to provide only a single response, practitioners identified bloodstain categories and patterns significantly more accurately than non-practitioners (p = 0.0001, p < 0.00001, respectively). Practitioner accuracy in bloodstain pattern recognition was positively associated with experience level (p = 0.0429) and this was consistent regardless of response scenario. Although no significant difference in practitioner accuracy was observed across response scenarios, practitioner conservatism/certainty varied significantly among the broad bloodstain category and specific pattern types. Overall, these results support bloodstain pattern recognition as an area of expertise and that practitioner experience positively influences accuracy. Based on these results, a series of recommendations were proposed aimed at further improving practices within the discipline to maximize accuracy and reliability of BPA evidence.