Causes and Causal Factors can usually be characterized as having two distinct but related qualities, termed Necessary and Sufficient.
Necessary: If x is required for y to occur, then y cannot occur unless x is present; x is a necessary cause of y. If evidence that y occurred is found, then x must have been present at some time. However, the the presence of x on its own does not imply that y did (or will) occur.
Sufficient: If x alone is required for y to occur, then y will occur when x is present; x is a sufficient cause of y. If evidence that y occurred is found, then x may have been present at some time. However, it is also possible that a different cause z was responsible for the presence of y.
Often in Root Cause Analysis, groups of causal factors may be considered together when examining causation. Each causal factor must have been necessary, or the event would not have occurred. In addition, all of the causal factors must have been present at the same location and at the same time for the event to occur. Thus, we say that the event was caused by a sufficient set of necessary causal factors; alternatively we can just refer to the set as the necessary and sufficient causes.
See all uses of the phrase Necessary and Sufficient on this site.
by Bill Wilson