 Absent any other compelling evidence to the contrary, it should initially be very likely that the defendant, as anyone else, had little to do with the crime. Say, for illustrative purposes, that the prior probability \(\Pr(\neg \source)= .99\) and so \(\Pr(\source)=.01\) (more on how to assess prior probabilities later in  §1.4).  Next, \(\Pr(\match \pmid \source)\) can be set approximately to \(1\), because if the defendant were the source of the blood at the crime scene, he should match the blood at the scene (setting aside the possibility of false negatives). The expert estimated the probability that someone who is not the source would coincidentally match the blood found at the scene—that is, \(\Pr(\match \pmid \neg \source)\)—as equal to .05. By Bayes’ theorem, 