DNA blunders sow doubts - academic

A growing body of wrongful convictions involving botched DNA testing is casting doubt on the "gold standard" of forensic science in courtrooms, a visiting United States academic says.

Though many cases involving DNA evidence were "clear cut", there was a disturbing number that were inaccurate, William Thompson, of California University's criminology, law and society department, told a Wellington conference.

Thompson, whose investigations into DNA evidence have helped to overturn wrongful convictions in Australia and the US, told Victoria University's Innocence Project that he "saw errors all the time".

He outlined cases in which DNA had been mishandled or cross-contaminated, samples had been mixed or false matches made. These were often discovered only after a conviction was re-examined.

"There is a strong tendency for people, even scientists in white coats, to interpret data in the light of what we expect (to see)."

There were also instances of poorly run forensic laboratories, including the Houston Police Department Crime Laboratory, which was closed for four years after concerns were raised about flawed work.

Retired High Court judge Sir Thomas Thorp, who investigated the Peter Ellis Christchurch creche case for the Government, estimates there could be around 20 people wrongly imprisoned in New Zealand.

About 1 per cent of the prison population was likely to be innocent, he said. "When you've got (roughly) 8000 people in prison, 1 per cent is quite a lot."

Thorp said many lawyers lacked the technical expertise to analyse and challenge dna evidence.

He recommended two years ago that an independent panel, similar to Britain's Criminal Cases Review Commission, be set up to investigate poten-ial miscarriages of justice.

This followed his research into 53 applications to the Justice Ministry claiming miscarriages of justice between 1995 and 2002, of which 26 per cent raised issues that "clearly required careful investigation", he said at the time.

Cases such as that of David Dougherty, whose rape conviction was quashed in 1997 after evidence established that semen in the victim's underwear was not his, showed New Zealand was not immune.

But Keith Bedford, Institute of Environmental Science and Research forensics programme manager, said DNA evidence had overturned more wrongful convictions than it had been responsible for.

He said human error was inevitable, but a double-checking procedure limited this and mistakes were recorded and acted on.

- © Fairfax NZ News