When the occurrence of one event depends of the occurrence of one or several other events, these events are said to be dependent. Therefore, when the dependencies are not taken under consideration, the results are underestimated. As they are no longer conservative, this cannot be acceptable, especially for safety studies. This is why the concepts of common-cause failure and common-mode failure have been introduced.
Source: API STD 689, Collection and Exchange of Reliability and Maintenance Data for Equipment, First Edition, July 2007. Global Standards