On the ‘‘post-dictive use’’ of the fault tree method for accident investigation to aid judicial procedures

This paper describes a methodology for accident assessment in the frame of a judicial procedure and provide the formal mathematical relations for the Fault Tree application to the case. The predictive use of Fault Tree (FT) technique is well known: the best estimate of an undesirable event (the top event, TE) frequency is computed from all the basic frequencies of component failures, human errors, and external events. The FT use is less frequent in “post-dictive” mode: in situations for which the TE is verified, as in the case of a judicial procedure. In such circumstances, several experts are appointed by the judge (JE) and by the parties, the plaintiffs and defendants (PEs), to reconstruct the event by following a top-down procedure analogous to that necessary for FT construction. They start from the event and try to find out the causes of the event, using all the circumstantial evidences. PEs often form different hypotheses, reaching contrasting conclusions, strongly influenced by the part represented, with JE in a more balanced position. It is so crucial to adopt a methodology that helps the experts, in particular the JE, to delimit, as much as possible, the area of the uncertainties. FT may be useful for this purpose. In fact, the FT construction allows to incorporate the evidences acquired from the investigations at the level of basic events: the basic events, now, are qualified as “true”, “false” or “unknown” (in the predictive contest, they are quoted through frequencies). In this way, it is possible to select the sequences accountable for the TE. In this paper a formal development for FT application to judicial procedure is given, together with a real case study. The method can be applied to all kind of accidents, provided that the situation can be described by a Fault Tree.