Using the structure of d-connecting paths as a qualitative measure of
  the strength of dependence
  Pearls concept OF a d - connecting path IS one OF the foundations OF the
modern theory OF graphical models : the absence OF a d - connecting path IN a
DAG indicates that conditional independence will hold IN ANY distribution
factorising according TO that graph. IN this paper we show that IN singly -
connected Gaussian DAGs it IS possible TO USE the form OF a d - connection TO
obtain qualitative information about the strength OF conditional
dependence.More precisely, the squared partial correlations BETWEEN two given
variables, conditioned ON different subsets may be partially ordered BY
examining the relationship BETWEEN the d - connecting path AND the SET OF
variables conditioned upon.
