Forensic science and the principle of excluded middle: “Inconclusive decisions” and the structure of error rate studies

In a paper published recently in this journal, Dror and Scurich (2020) [20] critically discuss the notions of “inconclusive evidence” (i.e., test items for which it is difficult to render a categorical response) and “inconclusive decisions” (i.e., experts’ conclusions or responses) in the context of forensic science error rate studies. They expose several ways in which the understanding and use of “inconclusives” in current forensic science research and practice can adversely affect the outcomes of error rate studies. A main cause of distortion, according to Dror and Scurich, is what they call “erroneous inconclusive” decisions, in particular the lack of acknowledgment of this type of erroneous conclusion in the computation of error rates. To overcome this complication, Dror and Scurich call for a more explicit monitoring of “inconclusives” using a modified error rate study design. Whilst we agree with several well-argued points raised by the authors, we disagree with their framing of “inconclusive decisions” as potential errors. In this paper, we argue that referring to an “inconclusive decision” as an error is a contradiction in terms, runs counter to an analysis based on decision logic and, hence, is questionable as a concept. We also reiterate that the very term “inconclusive decision” disregards the procedural architecture of the criminal justice system across modern jurisdictions, especially the fact that forensic experts have no decisional rights in the criminal process. These positions do not ignore the possibility that “inconclusives” – if used excessively – do raise problems in forensic expert reporting, in particular limited assertiveness (or, overcautiousness). However, these drawbacks derive from inherent limitations of experts rather than from the seemingly erroneous nature of “inconclusives” that needs to be fixed. More fundamentally, we argue that attempts to score “inconclusives” as errors amount to philosophical claims disguised as forensic methodology. Specifically, these attempts interfere with the metaphysical substrate underpinning empirical research. We point this out on the basis of the law of the excluded middle, i.e. the principle of “no third possibility being given” (tertium non datur).