doi:10.4156/ijact.vol2. issue5.8 Fault conditions can be used to analyze existing testing methods. The hierarchy of fault classes may not only explain experimental results of various testing methods, but also help to design effective test cases. We extend the known hierarchy to a wider range of fault classes based on detection conditions and prove the relationships between them. As a result, we show that for fault classes discussed in this paper test generation methods should give priority to the detection of variable insertion faults and missing expression faults