Beyond C.S.I.: The Rise of Computational Forensics
The DNA Difference
In stark contrast to the other forms of forensics evidence, DNA is considered extremely reliable, with very low error rates. Since 1989, DNA evidence has led to more than 200 exonerations, most involving wrongful convictions for rape and murder.
What accounts for this difference? In any forensics analysis, there are two basic issues: rarity and similarity. Let's say that the person who committed a crime is 220 centimeters tall. That's a fairly rare height for a human, so if you find a suspect of exactly that height, you'd consider that significant. But how sure are you that the perpetrator is 220 cm tall, and not 205 or 240 cm? Suddenly, the striking similarity in the data isn't quite so striking.
DNA analysis, which evolved serendipitously from genomic research, deals only with rarity. It starts with the fact that the human genome consists of 3 billion base pairs of nucleotides, which are stored within each cell's nucleus as well as in certain organelles outside the nucleus. Only about 1.5 percent of the nuclear genome contains useful information. The rest, known as junk DNA, has no survival function but is useful for identification because its sequence of base pairs varies from person to person.
A DNA profile is constructed by looking at the specific arrangement of a few hundred base pairs at 13 locations within a suspect's genome and then comparing the results with a known sample gathered at the crime scene. The entire process is automated. For each location, the probability of a random correspondence, or PRC—that is, the odds that two randomly chosen samples will have the same measured value—is 0.1. Therefore, the average PRC for all 13 locations is 1 in 10 trillion. Other forensic techniques may claim PRCs that low, but human examiners must get involved in those cases, and there is still too much uncertainty and bias in how they interpret the data.
Given the low error rate with DNA, why isn't it used in every criminal investigation? Because DNA is recovered from only about 10 percent of crime scenes. What's more, the mere presence of a suspect's DNA at the scene says nothing about how it got there. And jurors may not be swayed by DNA alone; often they want additional evidence that they can more easily comprehend, such as fingerprints, shoe prints, or handwriting. —Sargur N. Srihari