Les paramètres clés de l'interprétation des fibres textiles en sciences criminelles. Partie I: Occurrence et bruit de fond

ABSTRACT The question at the centre of forensic expertise is the determination of the probative value of a correspondence between recovered trace evidence and suspect material. In the field of textile fibers, it is necessary to decide on the meaning of non-differentiation between a textile in question and a reference group (of control) fibres from a putative source. To do this, it is possible to use a probabilistic approach, in particular a Bayesian approach, in order to assign a likelihood ratio to the observations resulting from laboratory examinations. Several parameters have to be considered: the rarity of the observed fibres (occurrence), the probability that the latter will come from the criminal action (transfer and persistence) and the probability that they are present by chance (not related to the criminal activity). The present article is a critical review of the sources of information published in the forensic literature that can be used to estimate the parameters for the calculation of the likelihood ratio. The main results are presented and are discussed in the light of the advantages and limitations of different types of studies. In this first part, only the studies dedicated to the determination of the scarcity of the index fibres (occurrence) and the ‘background’ fibres will be presented. Work on the transfer, persistence and recovery of fibres will be dealt with in Part Two.