Expert Witness by Anna Sandiford

involving physical fits, it can often be a simple case of yes, the pieces fitted together or no they didn’t — conclusive either way, no grey areas. For other areas of forensic science it’s a bit more complicated. DNA, for example, has a complex interpretation method based on statistical interpretation, which is part of the reason why DNA reporting in Victoria, Australia, was suspended at the latter end of 2009, temporarily halting the use of DNA in criminal cases because of a problem with interpretation of results after new technology was broughtonline in September. As a result of the new technology, more detailed information was obtained from DNA samples but the statistical models used to interpret the data were inadequate. In simple terms, it meant the DNA profiles could result in the wrong people being arrested, tried and convicted — miscarriage of justice, everyone’s worst nightmare.
    So who has checked all the databases that have been built up by police and prosecution agencies over the years? Who checks them to make sure the data is correctly entered or that the statistical basis for the interpretation is still correct and appropriate? As it turns out, the answer seems to be no one. An article in New Scientist entitled ‘Unreliable evidence? Time to open up DNA databases’ addressed this very issue. Most of the world’s DNA results (and I am talking about those relating to criminal casework) are interpreted using the results of relatively small studies undertaken during the early years of DNA forensic casework. The opening two paragraphs of the article cover it quite nicely, I think:
    When a defendant’s DNA appears to match DNA found at a crime scene, the probability that this is an unfortunate coincidence can be central to whether the suspect is found guilty. The assumptions used to calculate the likelihood of such a fluke … are now being questioned by a group of 41 scientists and lawyers based in the US and the United Kingdom. These assumptions have never been independently verified on a large sample of DNA profiles, says the group.
    I find that slightly unnerving, because I know from professional experience that if a DNA result is presented in court, it’sunbelievably difficult to shake anyone’s faith in the result. If the result is correct then there’s no problem; we just need to make sure that it is correct.
    Ignoring any other kind of DNA database for the moment, there are essentially two types of forensic DNA databases. The first is the crime scene database, which is the one that contains DNA profiles obtained from crime scene samples (so we don’t know who is the source of the DNA). The second is the reference sample DNA database, which contains the results of samples taken from known people, usually suspects (depending on the legal jurisdiction). I’m not suggesting there is anything significantly wrong with any of the DNA databases but they should be open to examination by independent scientists, so the reliance placed on DNA interpretations can be shown to be well-placed. It makes the results relevant and reliable, which is important for acceptance as evidence in court. Plus we don’t want situations where the wrong person goes to prison because of an incorrect match. The converse is that if a mistake was made entering a reference profile into a database, a comparison of a crime scene sample result with the reference samples might not show a match — we’d miss what could otherwise have been a ‘hit’.
    The big question is whether the FBI will allow a group of independent scientists to review the USA’s CODIS database — and the answer at the moment is no. I can under stand their reluctance in a way but I think it would be a better thing to have it reviewed now rather than wait for a specific case to demonstrate a monumental stuff-up and be beaten with the consequences later. It would also be better to have the