Importance of the causal structure. Solid line has direct causal influence ("direct" means that causal influence is not mediated by any other variable that is included in the study). Dashed line indicates there are two possibilities, but we do not know which holds. Dashed line without any arrow heads indicates there is a dependency but we do not know the reciprocal dependence.
Zhao et al. BMC Proceedings 2007 1(Suppl 1):S52