When two sets of evidence imply the same conclusion, there is a need to compute the total certainty factor based on the certainties of the sets of evidence.

A CF combination method should be:

- Commutative:
*A · B = B · A* - Associative:
*A · (B · C) = (A · B) · C*

This will make the resulting CF independent of the order in which pieces of evidence are considered.