Software trustability analysis

Abstract

A measure of software dependability called trustability is described. A program p has trustability T if we are at least T confident that p is free of faults. Trustability measurement depends on detectability. The detectability of a method is the probability that it will detect faults, when there are faults present. Detectability research can be used to characterize conditions under which one testing and analysis method is more effective than another. Several detectability results that were only previously described informally, and illustrated by example, are proved. Several new detectability results are also proved. The trustability model characterizes the kind of information that is needed to justify a given level of trustability. When the required information is available, the trustability approach can be used to determine strategies in which methods are combined for maximum effectiveness. It can be used to determine the minimum amount of resources needed to guarantee a required degree of trustability, and the maximum trustability that is achievable with a given amount of resources. Theorems proving several optimization results are given. Applications of the trustability model are discussed. Methods for the derivation of detectability factors, the relationship between trustability and operational reliability, and the relationship between the software development process and trustability are described.