Barriers to the Use of Fingerprint Evidence in Court Unlocked by Statistical Model
Fingerprints that are potential key pieces of evidence in court currently are not being considered due to shortcomings in the way this evidence is reported, according to a report by a Penn State Assistant Professor of Forensic Science and Statistics Cedric Neumann, published today in Significance, the magazine of the Royal Statistical Society and the American Statistical Association. Neumann has devised a statistical model to enable the weight of fingerprint evidence to be expressed in quantitative terms, paving the way for its full inclusion in the criminal-identification process. A detailed research paper describing the study will be published later this year in the Journal of the Royal Statistical Society: Series A.
Fingerprints have been used for over a century as a way of identifying criminals; however, fingerprint evidence is not currently permitted to be reported in court unless examiners claim absolute certainty that a mark has been left by a particular suspect. This courtroom certainty is based purely on the opinion of experts, formed through years of training and experience, but not on scientific data. Less-than-certain fingerprint evidence is not reported at all, without regard for the potential weight and relevance of the evidence in a case.
In Neumann's report in today's issue of Significance, he highlights the subjectivity in current processes, calling for changes in the way such key evidence is allowed to be presented. "It is unthinkable that such valuable evidence should not be reported, effectively hidden from courts on a regular basis. Such is the importance of this wealth of data, we have devised a reliable statistical model to enable the courts to evaluate fingerprint evidence within a framework similar to that which underpins DNA evidence," he said.
Credit: Frettie, via Wikimedia Commons
Neumann and his team devised and successfully tested a model for establishing the probability that a print could belong to a particular suspect. After mapping the finer points of detail on a "control print" from the suspect and a print obtained as evidence at the crime scene, the researchers tested two hypotheses. The first test -- designed to establish the probability that the crime-scene print was made by the suspect -- compared the control print with a range of other prints made by the suspect. The second test -- to establish the probability that the crime-scene print was made by someone other than the suspect -- compared the crime-scene print with a set of prints in a reference database. Neumann's research team then calculated the likelihood ratio between the two probabilities. The higher the ratio indicating stronger evidence that the suspect was the source of the crime scene print.
"Current practice allows a state of certainty to be presented in court which is not justified scientifically, or supported by logical process or data," Neumann said. "We believe that the examiner should not decide what evidence should or should not be presented. Our method allows all evidence to be supported by data, and reported according to a continuous scale."
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Neumann, C.: Fingerprints at the crime-scene: Statistically certain, or probable?; Significance (2012); DOI: 10.1111/j.1740-9713.2011.00539.x (URL: http://doi.wiley.com/10.1111/j.1740-9713.2011.00539.x)