Reflections on Glass Standards: Statistical Tests and Legal Hypotheses
Statistica Applicata - Italian Journal of Applied Statistics, Vol. 27, No. 2, pp. 173-186, 2015
15 Pages Posted: 27 Apr 2016 Last revised: 27 Dec 2016
Date Written: June 24, 2016
The past 50 years have seen an abundance of statistical thinking on interpreting measurements of chemical and physical properties of glass fragments that might be associated with crime scenes. Yet, the most prominent standards for evaluating the degree of association between specimens of glass recovered from suspects and crime scenes have not benefited from much of this work. Being confined to a binary match/no-match framework, they do not acknowledge the possibility of expressing the degree to which the data support competing hypotheses. And even within the limited match/no-match framework, they focus on the single step of deciding whether samples can be distinguished from one another and say little about the second stage of the matching paradigm — characterizing the probative value of a match. This article urges the extension of forensic-science standards to at least offer guidance for criminalists on the second stage of frequentist thinking. Toward that end, it clarifies some possible sources of confusion over statistical terminology such as “Type I” and “Type II” error in this area, and it argues that the legal requirement of proof beyond a reasonable doubt does not inform the selection of a significance level for tests of whether pairs of glass fragments have identical chemical or physical properties.
Keywords: hypothesis testing, matching, glass, forensic science, burden of proof
JEL Classification: C12
Suggested Citation: Suggested Citation