Cycles in the Influence Diagram
An Influence Diagram may not include cycles. There are two kinds of cycles: data and sequence, neither of which is permitted. These are defined below. An Influence Diagram may include circles, where a set of arcs leading out from a node eventually returns to that node.
A data cycle occurs when all the arcs in a circle are blue, green, or orange, indicating that the data for each node depends on the previous node. This is not permitted. Only black arcs do not indicate data relationships.
A sequence cycle occurs when all the arcs in a circle are blue, orange, or black, indicating that each node must follow the conditioning node. This is not permitted. Only green arcs, which indicate only probabilistic relationships, do not indicate sequence.
A circle with both a black arc and a green arc is not a cycle. Such a circle can occur when the Influence Diagram is used both to specify dependence and chronology.
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