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Forensic Evidence: The Reliability of DNA Testing
DNA testing analyzes a long chain of DNA, most if which is exactly the same among humans (and simians), and specific “loci” are found that differ among individuals. Courts have accepted the overall accuracy and value of DNA testing. For example, courts have allowed prosecutors to search for suspects by interviewing people in the DNA database who have merely similar DNA to that found at the crime scene, indicating family members.
However, exact probabilities of a match remain disputed. The FBI estimates that the odds of a coincidental match are 1 in 108 trillion. Other estimates are 1 in 113 billion, 1 in 10 billion, or 1 in 8192. To explain the variance, more and more loci are being discovered. Also, the DNA actually analyzed is but a chemical replication of the original. Statistics may also take into account human error and the probability of obtaining an uncorrupted DNA sample.
In a battle of statistics, prosecutors and defenders wage war, imploring the judge to allow certain statistics to be presented to the jury. Recently, the California Supreme Court addressed a “cold hit” murder case – where DNA at the crime scene was matched with a convict in the FBI database. The court allowed a “rarity statistic” to be told to the jury – that there was only a 1 in 930 sextillion chance of finding the same DNA profile in the general population.
Typically, a rarity statistic is derived from the normal police procedure of comparing the found DNA against every sample in the database. But the court left the door open for the defense to use the “database effect” or “birthday paradox,” which is when every entry is compared with every other entry. This would increase the probability of finding a match – for example, in a room of 57 people, there is a 99% chance of finding a matching birthday (which seems counterintuitive).
The statistics of molecular biology is an expert topic beyond the knowledge of most jurors. Just understanding what the numbers are saying can be difficult. Courts continue to grapple with what statistics would be proper to admit into evidence.
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