Forensic Metrology Case Study

There is no measurement that is 100% accurate in Forensic Science,

A measurement result is complete only when accompanied by a quantitative statement of its uncertainty. The uncertainty is required in order to decide if the result is adequate for its intended purpose and to ascertain if it is consistent with other similar results.

This case had a large resonance on the crime news in Italy, UK and US, due to the nationality of the involved people. A young British student, Meredith, attending the University in Perugia, Italy, was killed, in 2007, with several stab wounds inflicted by a kitchen knife.

The initial investigation led to the discovery of such a knife in the house of two school mates of Meredith, Raffaele, an Italian student, and Amanda, his American girl-friend. The coroners who examined the victim assessed that this knife was compatible with her wounds, so it could have been the murder weapon.

A DNA analysis was ordered and the report filed by the expert who was asked to perform the analysis stated that Raffaele’s and Amanda’s DNA were present on the knife handle (which was not surprising at all, since this was their kitchen knife), and that Meredith’s DNA was present on the blade. The report stated also that the quantity of biological material present on the knife blade was quite small and insufficient to repeat the test. The important point, here, is that uncertainty was estimated, but was reported without the necessary emphasis, so that it was not considered during the trial.

A DNA analysis was ordered and the report filed by the expert who was asked to perform the analysis stated that Raffaele’s and Amanda’s DNA were present on the knife handle (which was not surprising at all, since this was their kitchen knife), and that Meredith’s DNA was present on the blade. The report stated also that the quantity of biological material present on the knife blade was quite small and insufficient to repeat the test. The important point, here, is that uncertainty was estimated, but was reported without the necessary emphasis, so that it was not considered during the trial.

As a consequence of this test and the way it was reported to the trier of facts, the presence of Meredith’s DNA on the blade of Amanda’s and Raffaele’s knife was considered conclusive evidence that Amanda and Raffaele killed Meredith, and they were convicted of murder and sentenced to twenty-six years imprisonment.

The defence appealed against this verdict, and a second trial started in front of a different judge. According to the Italian law system, the judge of the second trial can order the repetition of the experimental tests considered during the first trial or, if this is not possible, a new analysis of the results can be requested. Since the repetition of the DNA analysis was not possible, the judge asked a team of experts to re-analyze the report filed by the expert who performed the DNA analysis and provide their opinion on the results.

This team considered the estimation of measurement uncertainty done by the expert who performed the DNA analysis and discussed it and the different contributions to uncertainty in deep details. The conclusion was that the values measured during the test were below the uncertainty value, and therefore, it was not possible to state whether the DNA on the blade was that of Meredith beyond the reasonable doubt quantified by measurement uncertainty. Surprisingly enough, this conclusion was already present, though not clearly exposed, in the original report, but was totally ignored by the trier of facts of the first trial.

The judge accepted the fact that the DNA test could not be considered conclusive evidence and returned a verdict of not guilt for Amanda and Raffaele, who were released after four years of imprisonment. Unfortunately, this was not the end of the story, because three more proceedings were celebrated before a final verdict of not guilt was returned.

For the sake of brevity, we do not consider, here, the judicial details of these proceedings, since, in the end, they confirmed the verdict of the first appeal trial.

On the other hand, it is worth noting that the absence of a clear indication

about measurement uncertainty led the trier of facts to assign the DNA test the importance of fully certain conclusive evidence, thus rendering a verdict beyond any reasonable doubt, whilst the doubt existed and was way more than reasonable, as recognized by the judge of the appeal trial and those of the Italian Supreme Court (Corte di Cassazione) who, finally, accepted those conclusions and rendered a verdict of not guilt.

In other words, measurement uncertainty, in this case, represented a major and important piece of evidence, and having neglected it led to an unfair verdict toward the two defendants.

Source: Forensic Metrology: Its importance and evolution by Ferrero and Scotti