Abstract
In a number of recent cases in the UK, convictions have been quashed by the Court of Appeal on the grounds that the jury had been misdirected as to the factual significance of random occurrence statistics. The mathematical basis on which those statistics are calculated was reviewed and recent appeal cases involving DNA evidence in the UK and the US were examined. It was found that a widespread misconception exists regarding the random occurrence ratio and its relationship with probability of guilt. It is in fact impossible to relate the two with any degree of accuracy without consideration of social and demographic factors particular to a case as well as any non-DNA evidence obtained.
http://linkinghub.elsevier.com/retrieve/pii/S0379073802001986
Also on complexity of evidence being out to a jury. (Malaysia has abolished jury trial long since)
http://www.ornl.gov/sci/techresources/Human_Genome/publicat/judicature/article10.html
Carole McCartney
DNA evidence is a powerful investigative tool, able to incriminate as well as exculpate. Yet, increasingly common portrayals of DNA as being able to solve crimes almost instantaneously, beyond any doubt, even from ‘beyond the grave’, may overstate the degree to which DNA currently assists in criminal investigations. Strong government support, and financial investment in the DNA Expansion Programme, have been bolstered by repeated legislative extensions of police powers to obtain and retain DNA samples. Despite this, DNA evidence remains marginal in terms of assisting with overall criminal detections and experts now suggest that the massive National DNA Database expansion has not resulted in the improvement in detection rates originally anticipated. This paper also suggests potential concerns over the ‘tactical’ use of DNA evidence during suspect interviews, and the risk of abbreviated police investigations. Insufficiently ‘forensically aware’ police officers may resort to DNA evidence in lieu of proper detective work, with literature on ‘case construction’ informing analysis of potential pitfalls of early reliance on DNA results, which may increase the risk ‘tunnel vision’ in criminal investigations.
http://bjc.oxfordjournals.org/cgi/content/abstract/46/2/175
more on DNA
The value of DNA evidence has to be seen in light of recent cases where criminals planted fake DNA samples at crime scenes. In one case, a criminal even planted fake DNA evidence in his own body: Dr. John Schneeberger of Canada raped one of his sedated patients in 1992 and left semen on her underwear. Police drew Schneeberger's blood and compared its DNA against the crime scene semen DNA on three occasions, never showing a match. It turned out that he had surgically inserted a Penrose drain into his arm and filled it with foreign blood and anticoagulants.(emphasis mine)
Evidence from an expert who has compared DNA samples must be accompanied by evidence as to the sources of the samples and the procedures for obtaining the DNA profiles. The judge must ensure that the jury must understand the significance of DNA matches and mismatches in the profiles. The judge must also ensure that the jury does not confuse the 'match probability' (the probability that a person that is chosen at random has a matching DNA profile to the sample from the scene) with the 'likelihood ratio' (the probability that a person with matching DNA committed the crime). In R v. Doheny, EWCA Crim 728 (1996). Phillips LJ gave this example of a summing up, which should be carefully tailored to the particular facts in each case:
Members of the Jury, if you accept the scientific evidence called by the Crown, this indicates that there are probably only four or five white males in the United Kingdom from whom that semen stain could have come. The Defendant is one of them. If that is the position, the decision you have to reach, on all the evidence, is whether you are sure that it was the Defendant who left that stain or whether it is possible that it was one of that other small group of men who share the same DNA characteristics.
Juries should weigh up conflicting and corroborative evidence, using their own common sense and not by using mathematical formulae, such as Bayes' theorem, so as to avoid "confusion, misunderstanding and misjudgment".
http://en.wikipedia.org/wiki/DNA_fingerprinting#Fake_DNA_evidence
