Computers are being used to automate critical services, including manufacturing systems, transportation, etc. For these critical applications, it is necessary to be able not only to achieve high quality but also to rigorously demonstrate that high quality has in fact been achieved. One approach that is used to facilitate prevention as well as detection of software faults is to decompose the requirements specification into more manageable portions. However, this does not necessarily enable the demonstration of high quality.

This paper discusses a method of decomposing software into aspects that allows the system reliability to be inferred from the aspect reliabilities. Each aspect is independently developable, i.e., it can be designed and implemented independently of the other aspects in the system. In addition, each aspect is end-user assessable, i.e., it can be tested or verified by the end-user independently of any other aspect. We identify five classes of IDEAL (Independently Developable End-user Assessable Logical) aspects and, for each class, we present the conditions that must be satisfied in order to compute the system reliability from the aspect reliabilities.