  Remarkably, a high percentage of naturally occurring computational problems turn out to be complete for one of the above classes. (A problem, \(A\), is complete for a complexity class \(C\) if \(A\) is a member of \(C\) and all other problems \(B\) in \(C\) are no harder than \(A\), i.e., \(B \le A\). Two complete problems for the same class have equivalent complexity.)