Crime statisticsStatistical model removes barriers to using fingerprint evidence in court

Potentially important fingerprint evidence is currently not being considered in legal proceedings owing to shortcomings in the way it is reported, according to a report published Wednesday in Significance, the magazine of the Royal Statistical Society and the American Statistical Association. Researchers involved in the study have devised a statistical model to enable the weight of fingerprint evidence to be quantified, paving the way for its full inclusion in the criminal identification process.

A Wiley release reports that fingerprints have been used for over a century as a way of identifying criminals. Fingerprint evidence, however, 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 categorical personal opinion, formed through years of training and experience, but not on logic or scientific data. Less-than-certain fingerprint evidence is not reported at all, irrespective of the potential weight and relevance of this evidence in a case.

Today’s Significance paper, which publishes in advance of the full study in the Journal of the Royal Statistical Society: Series A (forthcoming, vol. 9, no. 1[February 2012]), highlights this subjectivity in current processes, calling for changes in the way such key evidence is allowed to be presented. According to Professor of Statistics Cedric Neumann, “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.”

The release notes that Neumann, from Pennsylvania State University, and his team devised and successfully tested a model for establishing the probability of a print belonging to a particular suspect. After mapping the finer points of detail on a “control print” and “crime scene print”, two hypotheses were then tested. The first test, to establish the probability that the crime scene print was made by the owner of the control print (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. A likelihood ratio between the two probabilities was calculated; 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 which is not justified scientifically, or supported by logical process or data,” said Professor Neumann. “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.”

Neumann received his Ph.D. at the University of Lausanne, Switzerland, doing work on the interpretation of ink evidence. He collaborated on the International Ink Library project at the U.S. Secret Service (USSS) in Washington, D.C., in 2001, where he laid the foundations of the USSS Digital Ink Library. Between 2004 and 2010, he has also led a research team at the U.K. Forensic Science Service (FSS), developing statistical frameworks for assessing the weight of fingerprint evidence.

Neumann will give a presentation on current topics in statistics and forensic science on 31 July at the American Statistical Association’s Joint Statistical Meetings in San Diego, California.

— Read more in Cedric Neumann, “Fingerprints at the crime-scene: Statistically certain, or probable?” Significance 9, no. 1(8 February 2012): 21-25 (DOI: 10.1111/j.1740-9713.2012.00539.x); and in Carlos Gomez Grajales, “Stats vs Crime: As seen on TV,” Significance