* This manuscript, originally appearing as a Nov 1995 Microsoft
Research tech report, is a precursor to the Annals Oct 2002 publication
(arXiv:2105.03248) with material on distribution equivalence that was omitted
from the 2002 publication. This version contains corrections and updates to
the 1995 tech report Access Paper or Ask Questions
* Original version published in Learning in Graphical Models, M.
Jordan, ed., MIT Press, Cambridge, MA, 1999 * Addresses errors in the section on learning causal relationships Access Paper or Ask Questions