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When an FBI examiner misstates DNA evidence, what should happen?

Although DNA evidence can seem very convincing, forensic examiners make mistakes. They can run the test incorrectly. They can miss cross-contamination by other DNA. They can misstate the scientific certainty of their findings.

When an FBI forensic examiner makes such a mistake, it can wrongly convince a jury that the DNA evidence in the case is stronger than it actually is.

In Darius Caldwell's case, there is no question that an FBI examiner made a mistake that could have wrongly convinced a jury of his guilt. The question on appeal is whether that mistake was bad enough to justify a new trial.

Caldwell was convicted of two Georgia bank robberies that occurred in 2016. The bank robber wore a dreadlock wig and a face mask made out of a bandana and also carried a gun. The examiner claimed that DNA matching Caldwell was found on these items.

The examiner used the FBI's recommended language for testifying about the likelihood ratio, or the probability that a particular suspect is the source of the DNA evidence. But she made a mistake -- she used the word "if" when the recommended language said "that."

The difference in wording may have had a huge effect:

  • "It was 480,000 and 4.6 million times more likely that Caldwell was a contributor"
  • It was "480,000 and 4.6 million times more likely if Caldwell was a contributor"

Based on this important deviation from the FBI's recommended language, Caldwell's attorney asked the 11th Circuit Court of Appeals for a new trial.

There were also other irregularities in the case. For example, the police drove a bank teller to the scene of Caldwell's arrest and had her identify him. This was done instead of a fair police lineup or photo array, as would be done in a usual case.

US Attorney says the mistake made no difference

"There is simply no statistical or practical significance to the deviations [the examiner] made," argued the US attorney, who is handling the appeal for the government.

"That can't be right," said one of the appellate judges. "You mean to tell me that the testimony didn't change in any way because of her use of words?"

The US attorney contended that the wording change merely made for an "awkward delivery"; it was too subtle to change the meaning.

But it might have changed the meaning in the ears of some jurors. They could have been convinced by the higher-sounding likelihood ratio. Most people aren't skilled in statistical probabilities and could be misled by such differences. If a jury could have been confused by the misstatement, that should be enough for a new trial.

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