The way in which the fire investigator approaches the use of witness information, and how a witness statement influences the investigator’s final conclusions, remains a subject of some controversy in the fire investigation community. NFPA 921 supports the use of witness information as a legitimate source of data to be analyzed, but provides conflicting information on just how a witness statement can and should be used by a fire investigator in forming an expert origin or cause conclusion.
Level of Certainty
For a fire investigator to testify in court in compliance with NFPA 921 that a fire started in a particular location, or that a particular ignition source started it, he must believe only that his conclusion is probably correct—i.e., more likely than not. The guide provides no reference point on which to base this certainty other than to say it should be greater than 50%. In a forensic domain, effectively absent any known rates of accuracy or error, how that 51% certainty is recognized—like most of the processes of modern fire investigation—is completely in the hands (and the mind) of the individual investigator.