When it comes to DNA matches, you can prove almost anything using statistics.

CHARLES RICHARD SMITH has learned the hard way that you can prove almost anything with statistics. In 2009 a disputed statistic provided by a DNA analyst landed him with a 25-year jail sentence.

Smith was convicted of a sexual assault on Mary Jackson (not her real name) in Sacramento, California, which took place in January 2006. Jackson was sitting in a parking lot when a stranger jumped into her truck and made her drive to a remote location before forcing her to perform oral sex on him. When police arrested Smith and took a swab of cells from his penis, they found a second person's DNA mixed with his own.

The DNA analyst who testified in Smith's trial said the chances of the DNA coming from someone other than Jackson were 1 in 95,000. But both the prosecution and the analyst's supervisor said the odds were more like 1 in 47. A later review of the evidence suggested that the chances of the second person's DNA coming from someone other than Jackson were closer to 1 in 13, while a different statistical method said the chance of seeing this evidence if the DNA came from Jackson is only twice that of the chance of seeing it if it came from someone else.

How can a single piece of DNA evidence generate such massive differences in the statistical weight assigned to it? Last week, a New Scientist investigation showed how different forensic analysts can reach very different conclusions about whether or not someone's DNA matches a profile from a crime scene. This week we show how, even when analysts agree that someone could be a match for a piece of DNA evidence, the statistical weight assigned to that match can vary enormously.

"Usually DNA evidence is pretty strong," says David Balding, a statistical geneticist at University College London, whose calculation puts the lowest probability on the link between Smith and Jackson. "My point is that the number juries are provided with often overstates the evidence. It should be a smaller number."


Link:
http://www.newscientist.com/article/mg20727743.300-how-dna-evidence-creates-victims-of-chance.html