  In classic cases, Reichenbach's principle implies that discriminating between
common causes and causality is unprincipled since the discriminative results
essentially depend on the selection of possible conditional variables. For some
typical quantum cases, K.Reid $et$ $al$.
\href{https://www.nature.com/articles/nphys3266}{[Nat. Phys. 11, 414 (2015)]}
presented the statistic $C$ which can effectively discriminate quantum common
causes and quantum causality over two quantum random variables (i.e., qubits)
and which only uses measurement information about these two variables. In this
paper, we formalize general quantum common causes and general quantum
causality. Based on the formal representation, we further investigate their
decidability via the statistic $C$ in general quantum cases. We demonstrate
that (i) $C \in \left[ { - 1,\frac{1}{{27}}} \right]$ if two qubits are
influenced by quantum common causes; (ii) $C \in \left[ { - \frac{1}{{27}},1}
\right]$ if the relation between two qubits is quantum causality; (iii) a
geometric picture can illuminate the geometric interpretation of the
probabilistic mixture of quantum common causes and quantum causality. This
geometric picture also provides a basic heuristic to develop more complete
methods for discriminating the cases corresponding to $C \in \left[ { -
\frac{1}{{27}},\frac{1}{{27}}} \right]$. Our results demonstrate that quantum
common causes and quantum causality can be discriminated in a considerable
scope.
