Interpretation of Evidence: The Key to Conveying Information to Court

Abstract The advent of new technologies such as DNA typing, the weight of scientific evidence in criminal trials of widespread publicity, and the proliferation of fictional and non-fictional works in popular media have contributed to making forensic science well known, although perhaps not as well understood, by the general public. One of the consequences of this popularisation of forensic science was a sharp change in the attitude of investigators, who increasingly tend to delegate to scientists the collection of information necessary to identify the perpetrator of the crime. However, the prominent focus on the search of biological traces or fingerprints, due to their high potential for the personal identification of the individuals present at the crime scene, somewhat fade the interest towards other kinds of evidence, such as trace evidence. This kind of evidence is in fact perceived by judges and lawyers as less informative, because they think that “all plastic items are the same”, i.e. that it is impossible to discriminate among mass produced items. The purpose of this paper is to stress that, with sound methods for interpreting evidence, it is possible to improve the communication between the scientist and the Court, and to show the real significance of the analytical results, in the context of the case. The analysis of the traces found on a knife used in a murder case were performed by optical microscopy, IR spectroscopy, and UV-visible spectroscopy. The interpretation of evidence was carried out according to a Bayesian approach. A description of the interpretation of evidence in a case in which fibres were the key evidence. It is shown that the key aspects for having a high value of the evidence are the circumstances of the case and the reconstruction of the events given by the prosecutor and by the defence, in addition of course to a sound analytical procedure. In other words, it is shown that in some cases the evidential value of fibres or other trace evidence can be very high, sometimes comparable to that of fingerprints or DNA: when properly interpreted, trace evidence can give key information for solving cases.