 In criminal cases, confusion about the two directions of conditional probability can lead to exaggerating the probability of the prosecutor’s hypothesis. Suppose an expert testifies that the blood found at the crime scene matches the defendant’s and it is 5% probable that a person unrelated to the crime—someone who is not the source of the blood found at the scene—would match by coincidence. Some may be tempted to interpret this statement as saying that the probability that the defendant is not the source of the blood is 5% and thus it is 95% probable that the defendant is the source. This flawed interpretation is known as the prosecutor’s fallacy, sometimes also called the transposition fallacy (Thompson and Schumann 1987). The 5% figure is the conditional probability \(\Pr(\match \pmid \neg \source)\) that, assuming the defendant is not the source of the crime scene blood \((\neg \source),\) he would still match \((\match).\) The 5% figure is not the probability \(\Pr(\neg \source \pmid \match)\) that, if the defendant matches (\(\match\)), he is not the source (\(\neg \source\)). By conflating the two directions and thinking that 