Model-based testing is a technique to design abstract tests from models that partially describe the system's behavior. Abstract tests are transformed into concrete tests, which include test input values, expected outputs, and test oracles. Although test oracles require significant investment and are crucial to the success of the testing, we have few empirical results about how to write them. With the same test inputs, test oracles that check more of the program state have the potential to reveal more failures, but may also cost more to design and create. This research defines six new test oracle strategies that check different parts of the program state different numbers of times. The experiment compared the six test oracle strategies with two baseline test oracle strategies. The null test oracle strategy just checks whether the program crashes and the state invariant test oracle strategy checks the state invariants in the model. The paper presents five main findings. (1) Testers should check more of the program state than just runtime exceptions. (2) Test oracle strategies that check more program states do not always reveal more failures than strategies that check fewer states. (3) Test oracle strategies that check program states multiple times are slightly more effective than strategies that check the same states just once. (4) Edge-pair coverage did not detect more failures than edge coverage with the same test oracle strategy. (5) If state machine diagrams are used to generate tests, checking state invariants is a reasonably effective low cost approach. In summary, the state invariant test oracle strategy is recommended for testers who do not have enough time. Otherwise, testers should check state invariants, outputs, and parameter objects.
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