Our new paper* "When ‘neutral’ evidence still has probative value: implications from the Barry George Case" (published in the journal Science and Justice) casts doubts on the reasoning in the 2007 Appeal Court judgement that led to the quashing of Barry George's conviction for the shooting to death of TV celebrity Jill Dando.
The paper examines the transcript of the Appeal in the context of new probabilistic research about the probative value of evidence. George's successful appeal was based primarily on the argument that the prosecution's evidence about a particle of firearm discharge residue (FDR) discovered in George's coat pocket, was presented in a way that may have misled the jury. Specifically, the jury in the original trial had heard that the FDR evidence was very unlikely to have been found if Barry George had not fired the gun that killed Jill Dando. Most people would interpret such an assertion as strong evidence in favour of the prosecution case. However, afterwards the same forensic expert concluded that the FDR evidence was just as unlikely to have been discovered if Barry George had fired the gun. In such a scenario the evidence is considered to be ‘neutral’ - favouring neither the prosecution nor the defence. Hence, the appeal court considered the verdict unsafe and the conviction was quashed. Following the appeal ruling, the FDR was excluded from the jury at George's retrial and he was acquitted. However, our paper shows that the FDR evidence may not have been neutral after all.
Formally, the probative value of evidence is captured by a simple probability formula called the likelihood ratio (LR). The LR is the probability of finding the evidence if the prosecution hypothesis is true divided by the probability of finding the evidence if the defence hypothesis is true. Intuitively, if the LR is greater than one then the evidence supports the prosecution hypothesis; if the LR is less than one it supports the defence hypothesis, and if the LR is equals to one (as in the case of the FDR evidence here) then the evidence favours neither and so is 'neutral'. Accordingly the LR is a commonly recommended method for forensic scientists to use in order to explain the probative value of evidence. However, the new research in the paper shows that the prosecution and defence hypotheses have to be formulated in a certain way in order for the LR to 'work' as expected. Otherwise it is possible, for example, to have evidence whose LR is equal to one but which still has significant probative value. Our review of the appeal transcript shows that relevant prosecution and defence hypotheses were not properly formulated and, if one were to follow the arguments recorded in the Appeal judgement verbatim, then contrary to the Appeal conclusion, the probative value of the FDR evidence may not have been neutral as was concluded, but rather still supported the prosecution**.
*Full details: Fenton, N. E., D. Berger, D. Lagnado, M. Neil and A. Hsu, (2013). "When ‘neutral’ evidence still has probative value (with implications from the Barry George Case)", Science and Justice, http://dx.doi.org/10.1016/j.scijus.2013.07.002 published online 19 August 2013. For those who do not have full access to the journal, a pre-publication draft of the article can be found here.
** Although the FDR evidence may have been probative after all, we are not in a position to comment on the overall case against Bary George, which others have argued was not particularly strong. Also, it could be argued that even though the FDR evidence was not 'neutral' as assumed in the Appeal, its probative value may not have been as strongly favourable to the prosecution as implied in the original trial; this may have been sufficient in itself to cast doubt on the safety of the conviction.