  Control Strategies for hierarchical tree-like probabilistic inference
networks are formulated and investigated. Strategies that utilize staged
look-ahead and temporary focus on subgoals are formalized and refined using the
Depth Vector concept that serves as a tool for defining the 'virtual tree'
regarded by the control strategy. The concept is illustrated by four types of
control strategies for three-level trees that are characterized according to
their Depth Vector, and according to the way they consider intermediate nodes
and the role that they let these nodes play. INFERENTI is a computerized
inference system written in Prolog, which provides tools for exercising a
variety of control strategies. The system also provides tools for simulating
test data and for comparing the relative average performance under different
strategies.
