Abstract: Various methods of generating test cases based on Boolean specifications have previously been proposed. These methods are fault-based in the sense that test cases are aimed at detecting particular types of faults. Empirical results suggest that these methods are good at detecting particular types of faults. However, there is no information on the ability of these test cases in detecting other types of faults. This paper summarizes the relationships of faults in a Boolean expression in the form of a hierarchy. A test case that detects the faults at the lower level of the hierarchy will always detect the faults at the upper level of the hierarchy. The hierarchy helps us to better understand the relationships of faults in a Boolean expression, and hence to select fault-detecting test cases in a more systematic and efficient manner.