One often hears the claim that mutation analysis is unreliable because of the presence of immortal (equivalent) mutants. The problem is that the true mutation score is the ratio between the number of mutants killed and the actual number of mortal mutants. Since there is no general algorithm to determine all immortal mutants from a set of mutants, it is impossible to determine the actual number of mortal mutants from an arbitrary set of mutants. Hence, the mutation score is unreliable as a measure of test suite quality, which is the primary purpose of mutation testing.
Further, live mutants are often used by practitioners to determine how to enhance their test cases so that it covers more specification by killing them. Immortal mutants, by definition can not help here because they can not be killed. Hence, human effort spent on analyzing these mutants is often considered a waste.
Our thesis is that one needs to consider the issue from a more ho