Mutation testing is a powerful, but resource intense technique for asserting software quality. This report investigates two claims about one of the mutation operators on procedural logic, the relation operator replacement (ROR). The constrained ROR mutant operator is a type of constrained mutation, which targets to lower the number of mutants as a “do smarter” approach, making mutation testing more suitable for industrial use. The findings in the report shows that the hypothesis on subsumption is rejected if mutants are to be detected on function return values. The second hypothesis stating that a test case can only detect a single top-level mutant in a subsumption graph is also rejected. The report presents a comprehensive overview on the domain of mutation testing, displays examples of the masking behaviour previously not described in the field of mutation testing, and discusses the importance of the granularity where the mutants should be detected under execution. The contribution is based on literature survey and experiment. The empirical findings as well as the implications are discussed in this master dissertation.