Technical support for a judge when assessing a priori odds

Abstract

The Engineering Department of the Spanish Civil Guard has been using automatic speaker recognition systems for forensic purposes providing likelihood ratios since 2004. They are quantitatively much more modest than in the DNA field. In this context, it is essential a suitable calculation of the prior odds to figure out the posterior odds once the comparison result is expressed as likelihood ratio. These odds are under the responsibility of a Judge, and many consider unlikely that they can be quantitatively calculated in real cases. However, our experience defending in Court over 500 speaker recognition expert reports allows us to suggest how the expert may support Judges from a technical point of view to assess the odds. Technical support as referred should be preferentially provided in the preliminary investigation stage, after the expert report being issued by the laboratory, as in the course of oral hearings it is much more difficult for those who are not familiar with the new paradigm. It can be initiated upon request by the Examining Judge or any of the litigant parties. We consider this practice favourable to the equality of arms principle.The use of Bayesian networks is proposed to provide inferential assistance to the Judge when assessing the prior odds. An example of the explanation above is provided by the case of the terrorist attack against Madrid-Barajas Airport Terminal 4 perpetrated in December 2006.