This is a very subjective question, and I don't believe that you should be putting your subjective feelings about your level of certainty into a report. We have been trying to address this in the OSAC, and here is a draft of what I have written about level of certainty.


Level of Certainty
NFPA 921 in the chapter on Basic Methodology addresses the level of certainty, and it states, “The level of certain certainty describes how strongly someone holds an opinion (conclusion). Someone may hold any opinion to a higher or lower level of certainty. That level is determined by assessing the investigator’s confidence in the data, in the analysis of that data and testing of hypotheses formed. That level of certainty may determine the practical application of the opinion, especially in legal proceedings.

NFPA 921 further states that many courts have set a threshold of certainty for the investigator to be able to rendering opinions in court such as “proven to an acceptable level of certainty,” “a reasonable degree of scientific and engineering certainty,” or “ a reasonable degree of certainty within my profession.” The NFPA 921 Technical Committee takes no position on this language and states that it is “beyond the scope” of the document.

The National Commission on Forensic Science (NCFS), on the other hand, has addressed the level of certainty that some lawyers and judges mistakenly believe is required for testimony. That is “a reasonable degree of scientific (or medical, or engineering, or discipline) certainty.” In fact, “scientific certainty” does not exist. It is a term of art coined by lawyers and is now falling into disrepute.

On March 22, 2016, the NCFS voted to recommend that the term “reasonable certainty,” whether couched as "scientific certainty or "[discipline] certainty, should not be used. It recommended that the Attorney General direct all attorneys appearing on behalf of the DOJ (a) to forgo the use of these phrases unless directly required by judicial authority as a condition of admissibility of the witness’s opinion and (b) to assert the legal position that such terminology is not required and is indeed misleading. The second recommendation to the Attorney General states that he should direct all forensic science service providers and forensic science medical providers employed by the DOJ not to use such language in reports or to couch their testimony in such terms unless directed to do so by the judge. A final recommendation adopted by the NCFS is that the Attorney General should, in collaboration with NIST, urge the OSACs to develop appropriate language that may be used by experts when reporting or testifying about results or findings based on observations of evidence and the data derived from the evidence. [www.justice.gov]


According to one legal scholar “The reasonable-degree-of-scientific-certainty language almost certainly was drafted by the lawyers. Scientists have no use for this phrase (outside the courtroom). Indeed, “a reasonable degree of scientific certainty” is not a defined concept in scientific disciplines or even in law. … It is legal mumbo-jumbo derived from archaic cases in which lawyers discovered that if a medical doctor did not utter the incantation “to a reasonable degree of medical certainty,” his testimony might be excluded because doctors were not supposed to talk about mere probabilities. (David H. Kaye, The Double Helix And The Law Of Evidence, Harvard University Press, 82 (2010)).



NFPA 921 defines two levels of confidence: “possible” and “probable,” and instructs fire investigators not to testify to opinions that are merely “suspected,” and to identify when they are testifying about opinions that are only “possible.” The problem with testifying to a “reasonable degree of scientific certainty” is that a jury might equate it with certainty at the level of beyond a “reasonable” doubt.