
90 Bayesian Networks: With Examples in R
(PDAG) obtained from dag’s skeleton and its v-structures must be completed
into a CPDAG by compelling the direction of X8 → X6, which is then called
a compelled arc. Similarly, if disregarding the direction of an arc would result
in one of its possible direction introducing cycles, such an arc is considered
compelled and its original direction preserved.
On the other hand, the direction of the arc X5 → X7 is not preserved in
the CPDAG. Neither X5 → X7 nor X7 → X5 introduce any v-structure in the
graph, as they are part only of serial (the former) or divergent connections
(the latter). The same holds for the