As in Table 3.4, we assume that there are 20 possible suspects, only one of whom is guilty. This provides the totals in the bottom row. The totals for each column are mult iplied by the probabilities in the appropriate cell of Table 3.5 to get the values in this table, which can be used to compute the probability that an innocent suspect is mistakenly identified as guilty based on DNA evidence. This probability is 0.0000019/0.9950019, which is much less than 1%, if the assumptions about the use of DNA evidence discussed in the text are correct.

and 4,999,999 innocent suspects, the upper-right cell of the table would become 0.0000001 X 4,999,999 = 0.5. Therefore the top row of the table would be changed to 0.995, 0.5, and 1.445. Given a match between DNA from a crime scene and DNA from one of the 5 million individuals in the database, the probability of guilt for that particular individual would be 0.995/1.445, or 66%. Just as in the example of the occult-stool test for col-orectal cancer, in which such a small proport ion of the populat ion has the disease that most false positive tests occur in healthy people, there is a substantial likelihood of a false positive result if DNA from a large number of innocent people is screened to find a match to DNA collected at a crime scene.

DNA evidence is certainly a powerful tool in forensic identification, but it can be misused in several ways, including the k ind of fishing expedit ion described in the last paragraph (Roeder 1994). Although the calculations often depend on assumptions that can't be verified (e.g., that the total number of possible suspect s is k nown), this is a valuable exercise because it introduces a systematic way of thinking about the credibil ity of evidence.

In this chapter we've examined several experiments dealing with the olfactory abilities of dogs in the context of forensic work. These experiments were not ideal in design and execut ion, but perhaps they are more valuable for learning some of the basic elements of experimentation because of their flaws than more rigorous experiment s would have been. It m ight seem that it would be easy to study the behavior of dogs experimentally because they are so much more familiar to us than other animals, but in fact studying trained dogs means work ing with their handlers as well, and this can complicate experiments significantly.

Although the limited experimentation that has been done to date, mostly by Gertrud Schoon, casts doubt on the validity of scent lineups in "proving" that a suspect committed a crime, some of the other work that trained dogs do is st ill credible. This includes finding people buried by avalanches or collapsed buildings, sniffing out hidden narcot ics, and tracking fugitives. A lso, dogs clearly have the sensory ability to identify odors specific to individuals and thus determine if a suspect in a scent lineup has the same odor as an item from a crime scene, although not infallibly. The key is to devise a training method that enhances the accuracy and reliability of this identification.

Finally, this chapter illustrates a quantitative method for evaluating evidence. I used examples ranging from diagnostic tests for disease to matching DNA from a crime scene to DNA of a suspect, but this method has even broader applicability to hypothesis testing in general. In many cases, we may not have all the information necessary to apply this method. However, using this approach as a framework for thinking about the meaning of evidence can help us develop an appropriate level of skepticism about how scientific information is used in pract ical matters, as well as in basic research. The result s of this approach are somet imes surprising, as in some of the examples we discussed. If jurors were better educated about these ideas, we would all benefit from more rational judicial proceedings.

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