Where did they go? The secondary (or higher) transfer of fibres from the home environment via daily wear

Abstract

When evaluating findings of trace-trace or trace-reference fibre comparisons that involve materials recovered from frequented environments, the probabilities of the secondary or higher transfer of these fibres need to be considered. Such a complex process of transfer can be roughly divided into three main parts, the acquisition of these fibres from the environment, followed by their persistence, and finally their re-transfer during the event of interest. The first step was explored in a previous article, while existing literature is already rich in data on fibre persistence. This study focuses on the third step of the process, particularly in assault scenarios. Two participants with known rates of fibre acquisition from their domicile environment were recruited to wear six shirts of two levels of surface roughness for eight hours in their home. These shirts (intermediates) were then collected and worn by combat athletes during an assault simulation by the volunteer playing the role of assailant. Meanwhile, the recipient wore a standardized blanked white t-shirt (recipients). Both sets of shirts were collected and analysed for target fibres identified from the home environment of the two participants. The quantity of these target fibres on each of these shirts were recorded and compared to the previously recorded acquisition rate of the two participants. Surface roughness was found to play a role in transfer, where a greater share of fibres was retained on the rougher intermediates. The class of fibre did not appear to influence the quantity transferred. Finally, the results were used to establish posterior models in four different scenarios that describe varying levels of information available to the forensic expert. These models could be roughly classified into count-based, which involved a direct modelling of the number of fibres transferred with a negative binomial-beta conjugate, and proportion-based, involving first the modelling of the acquisition using a negative binomial distribution, and the subsequent transfer with a binomial distribution, using the results of the first model as the size and a beta prior distribution to model the proportion transferred. Proportion-based models outperformed the count-based ones, and increasing levels of information permitted increased specificity towards each scenario.