Kathleen Z. Rooney
A much older technology than DNA, often considered as specific and individualized as DNA, fingerprinting also uses the multiplication principle of probability to hone in on a certain match. Fingerprint probabilities can be less easy to pinpoint than DNA for a couple of specific reasons. One reason is that fingerprints collected from crime scenes are often imperfect. A second reason is that the probability of a match at a particular locus may not as easily quantified as it is in DNA. Loci in fingerprints are determined by gridding the image and matching areas on the grid. The match is visual and based on an expert''s judgment call. However, some experts argue that fingerprints are more reliable, matching as many as 36 loci, called minutiae.
In 2004 a tremendous error involving probabilities can be used as an example to illustrate how a one in a million chance can actually happen one in a million times. This can be used as an introduction to randomness and the law of large numbers.
On March 11, 2004, coordinated attacks on the commuter trains in Madrid, Spain, wounded approximately 200 people and killed 190. In a sweeping search for a fingerprint match to a print found on a bag of explosives, FBI investigators honed in on a lawyer in Oregon in the United States. His arrest was based on an absolute certainty among the investigators, all of whom were highly skilled, that they had found a perfect match of the print. They identified 15 areas of similarity. The problem here was that they did not limit their search to suspects. The unrestricted search in addition to the size of the database, turned up a virtual twin. The lawyer was released two weeks later when the Spanish police found a strong match with an Algerian suspect.
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