Newspaper article Pittsburgh Post-Gazette (Pittsburgh, PA)

Know the Limits Forensic Evidence Deserves Legal Scrutiny, Too

Newspaper article Pittsburgh Post-Gazette (Pittsburgh, PA)

Know the Limits Forensic Evidence Deserves Legal Scrutiny, Too

Article excerpt

"The method was long hailed as infallible, despite the lack of appropriate empirical studies to assess its error rate." That's from the President's Council of Advisors on Science and Technology, referring to latent fingerprint analysis - checking to see whether fingerprints such as those at a crime scene match those given by a known person under controlled conditions. But the theme is pervasive in the report it released this week: The forensic sciences may be getting more credit than they are scientifically due. It's good policy to make sure all such disciplines are rigorously tested.

Investigators and courts rely on forensics to identify suspects, prove them guilty and exonerate them. Yet in many of the 342 cases where DNA has exonerated a defendant, the report says that "forensic scientists . told juries incorrectly that similar features in a pair of samples taken from a suspect and from a crime scene (hair, bullets, bite marks, tire or shoe treads, or other items) implicated defendants in a crime with a high degree of certainty."

The case of fingerprints is a good one. There really is evidence, PCAST says, that fingerprints can be reliably analyzed. But even there, one study PCAST mentions says the false-positive rate - the rate at which a match will be found with the wrong person - could be 1 in 18. And examiners often change their interpretation of a print based on comparison with a possible match, undermining their objectivity.

Even simple DNA analysis can go wrong because of human error. And when there are multiple DNA sources in a sample, the task of identifying them becomes much more complicated. Going by expert judgment in those complex cases hasn't been shown to be reliable, according to the report.

The analysis of bite marks to see whose teeth they came from and the analysis of bullets to see which gun they came from have not been proved scientifically valid, the report says. …

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