Belief networks provide an important bridge between statistical modeli
ng and expert systems. This article presents methods for visualizing p
robabilistic ''evidence flows'' in belief networks, thereby enabling b
elief networks to explain their behavior. Building an earlier research
on explanation in expert systems, we present a hierarchy of explanati
ons, ranging from simple colorings to detailed displays. Our approach
complements parallel work on textual explanations in belief networks.
Graphical-Belief, Mathsoft Inc.'s belief network software, implements
the methods.