They were looking for two types of validity. According to PCAST, foundational validity means the forensic discipline is based on research and studies that are “repeatable, reproducible, and accurate,” and therefore reliable. The next step is applied validity, meaning the method is “reliably applied in practice.” In other words, for a forensic discipline to produce valid evidence for use in court, there must be (1) reproducible studies on its accuracy and (2) a method used by examiners that is reproducible and accurate.
Among the forensic science they assessed, PCAST found single-sourced DNA analysis to be the only discipline that was valid, both foundationally and as applied. They found DNA mixture evidence – when DNA from more than one person is in a sample, for instance from the victim and the perpetrator, multiple perpetrators or due to contamination – to be only foundationally valid. Same with fingerprint analysis.
Firearms identification had just the potential for foundational validity, but the research that could support it hasn’t been done yet. Footwear analysis lacked studies even showing potential for foundational validity. And bite mark analysis has a low chance of achieving any validity; the PCAST report advised “against devoting significant resources” to it.
All these types of evidence are widely used in thousands of trials each year. Many additional cases never even go to trial because this supposedly definitive evidence seems damning and compels defendants to plead guilty.