An important aspect of developing models relating the number and type of faults in a software system to a set of structural measurement is defining what constitutes a fault. By definition, a fault is a structural imperfection in a software system that may lead to the system?s eventually failing. A measurable and precise definition of what faults are makes it possible to accurately identify and count them, which in turn allows the formulation of mod-els relating fault counts and types to other measurable attributes of a software system. Unfortunately, the most widely-used definitions are not measurable - there is no guarantee that two different individuals looking at the same set of failure reports and the same set of fault definitions will count the same number of underlying faults. The incomplete and ambiguous nature of current fault definitions adds a noise component to the inputs used in