 The directions of the arrows indicate that the accuracy of the evidence (accuracy node) and the alcohol level (hypothesis node) influence the outcome of the test (evidence node). The graphical model represents different sources of uncertainty. The uncertainty associated with the sensitivity and specificity of the test—that is, the probability that the tests reports excessive alcohol level when the level is excessive (sensitivity) and the probability that the test reports normal alcohol level when the level is normal (specificity)—is captured by the arrow going from the hypothesis node (Excess alcohol level) to the evidence node (Evidence for excess). Other sources of uncertainty comprise the possibility that the police officer lied about the test report or the possibility that the driver took medications which then affected the alcohol level. These possibilities can be taken into consideration by adding an accuracy node (or multiple accuracy nodes, if each factor is kept separate from the other).