The defendant appealed against his conviction for rape. There had been a DNA match, but the defendant did not match the description given by the victim, and she had not picked him out. He argued that DNA evidence alone should not be used to convict, and that the judge had misdirected the jury as to the use of statistical evidence. The court considered how DNA evidence might be presented to a jury.
Held: The appeal was allowed, and a decision was to follow as to a retrial.
The use of statistical theory in support of a case is to be discouraged in criminal trials: ‘To introduce Bayes Theorem, or any similar method, into a criminal trial plunges the Jury into inappropriate and unnecessary realms of theory and complexity deflecting them from their proper task.’
The court considered the error of seeking to use probability in a criminal court. Rose LJ said: ‘More fundamentally, however, the attempt to determine guilt or innocence on the basis of a mathematical formula, applied to each separate piece of evidence, is simply inappropriate to the jury’s task. Jurors evaluate evidence and reach a conclusion not by means of a formula, mathematical or otherwise, but by the joint application of their individual common sense and knowledge of the world to the evidence before them. It is common for them to have to evaluate scientific evidence, both as to its quality and as to its relationship with other evidence. Scientific evidence tendered as proof of a particular fact may establish that fact to an extent which, in any particular case, may vary between slight possibility and virtual certainty. For example, different blood spots on an accused’s clothing may, on testing, reveal a range of conclusions from ‘human blood’ via ‘possibly the victim’s blood’ to ‘highly likely to be the victim’s blood’. Such evidence is susceptible to challenge as to methodology and otherwise, which may weaken or even, in some cases, strengthen the impact of the evidence. But we have never heard it suggested that a jury should consider the relationship between such scientific evidence and other evidence by reference to probability formulas. That such a course would in any event be impossible of sensible achievement by a jury, at least so far as the use of the Bayes theorem is concerned, is demonstrated by the practical application of the stage of that theorem’s methodology that involves numerical assessment of the various items of evidence. Individual jurors might differ greatly not only according to how cogent they found a particular piece of evidence (which would be a matter for discussion and debate between the jury as a whole), but also on the question of what percentage figure for probability should be placed on that evidence. Since, as we have pointed out, the translation of an assessment of cogency into a percentage probability of guilt is entirely a matter of judgment and the conferring of a percentage probability of guilt upon one item of evidence taken in isolation is an essentially artificial operation, different jurors might well wish to select different numerical figures even when they were broadly agreed on the weight of the evidence in question. They could, presumably, only resolve any such difference by taking an average, which would truly reflect neither party’s view; and this point leaves aside the even greater difficulty of how twelve jurors, applying Bayes as a single jury, are to reconcile, under the mathematics of that formula, differing individual views about the cogency of particular pieces of evidence. ‘
Judges:
Rose, Hidden, Buxton LJJ
Citations:
Times 09-May-1996, [1996] EWCA Crim 222, [1996] 2 Cr App R 467, [1996] Crim LR 898, [1996] 2 Cr App Rep 467
Links:
Statutes:
Police and Criminal Evidence Act 1984 78
Jurisdiction:
England and Wales
Cited by:
Cited – Regina v Doheny, Adams CACD 31-Jul-1996
The court set out the procedure for the introduction of DNA evidence in criminal trials. In particular the court explained the ‘Prosecutor’s Fallacy’ when using statistical evidence. The significance of the DNA evidence will depend critically upon . .
Lists of cited by and citing cases may be incomplete.
Criminal Evidence
Updated: 08 October 2022; Ref: scu.86032