 The discussion so far has focused on the case where there are no latent common causes of the variables in \(\bV\), and the error variables \(U_i\) can be expected to be probabilistically independent. As we noted in  Section 2.3  above, we represent a latent common cause with a double-headed arrow. For example, the acyclic directed mixed graph in  Figure 9  represents a latent common cause of X and Z. More generally, we can use an ADMG like Figure 9 to represent that the error variables for X and Z are not probabilistically independent.