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The empirical assessment of test techniques plays an important role in software testing research. One common practice is to instrument faults, either manually or by using mutation operators. The latter allows the systematic, repeatable seeding of large numbers of faults; however, we do not know whether empirical results obtained this way lead to valid, representative conclusions. This paper investigates this important question based on a number of programs with comprehensive pools of test cases and known faults. It is concluded that, based on the data available thus far, the use of mutation operators is yielding trustworthy results (generated mutants are similar to real faults). Mutants appear however to be different from hand-seeded faults that seem to be harder to detect than real faults.

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The issue of ordering class integration in the context of integration testing has been discussed by a number of researchers. More specifically, strategies have been proposed to generate a test order while minimizing stubbing. Recent papers have addressed the problem of deriving an integration order in the presence of dependency cycles in the class diagram. Such dependencies represent a practical problem as they make any topological ordering of classes impossible. Three main approaches, aimed at "breaking" cycles, have been proposed. The first was proposed by Tai and Daniels and is based on assigning a higher-level order according to aggregation and inheritance relationships and a lower-level order according to associations. The second was proposed by Le Traon et al. and is based on identifying strongly connected components in the dependency graph. The third was proposed by Briand et al.; it combines some of the principles of the two previous approaches and addresses some of their shortcomings (e.g., the first approach may result in unnecessary stubbing whereas the second may lead to breaking cycles by "removing" aggregation or inheritance dependencies, thus leading to complex stubbing). This paper reviews these strategies (principles are described, advantages and drawbacks are precisely investigated), and provides both analytical and empirical comparisons based on five case studies.