DNA Test Results: Probability vs. Fallacy

The State of North Carolina goes to trial against Donnie Defendant, who is alleged to be the infamous “Tarheel State Killer” and charged with committing a series of brutal assaults and murders several decades ago. The state’s case depends heavily on matching DNA evidence from the crime scene to a sample of DNA taken off a cigarette butt discarded by Donnie. At trial, Special Agent Wanda Witness testifies as an expert in forensic DNA analysis for the state. After explaining the science behind PCR, STR, loci, and markers, Wanda opines that Donnie’s DNA is indeed a match to the DNA recovered from the crime scene. Sounds like good news for the state, but what exactly does a “match” mean? And how may the significance or statistical probability of that “match” be expressed to the jury? It’s an important question, because what might sound like two similar ways of expressing the same probability can have dramatically different meanings – and possibly even be considered error on appeal. Testimony and Argument at Trial Continuing her direct examination, Wanda testifies that the “random match probability” in this case is calculated to be 1 in 10,000,000. She explains that this means if one were to go out and select an unrelated person completely at random, and then compare that person’s DNA to the DNA sample taken from the crime scene, the likelihood that those two samples would happen to be a match is only one in ten million. Wow. Strong evidence. Paula Prosecutor thinks so [...]