THE PROBLEM OF MIXED DNA. The two types of problems with DNA that I described in the last two posts are similar to problems with other kinds of forensic evidence. First, keeping a chain of evidence and avoiding contamination are always necessary. Second, eliminating alternative sources of touch DNA is similar to the general problem of eliminating alternative explanations. A third kind of error is peculiar to DNA. Because of the accuracy of DNA when there are good samples left at a crime scene, it is easy for jurors to underrate the difficulties posed by small samples of DNA and mixtures of two or more sources of DNA.

Matthew Shaer gives an example which “illustrates exactly how subjective the reading of complex mixtures can be.” The issue was whether a defendant could be excluded as a contributor to a mixture of sperm. At trial,two forensic scientists had concluded that the defendant could not be excluded as a contributor. When the issue was presented to 17 lab technicians, only one of the 17 lab technicians concurred that the defendant could not be excluded as a contributor.

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