A quasi-renewal process for software reliability and testing costs

This paper models software reliability and testing costs using a new tool: a quasi-renewal process. It is assumed that the cost of fixing a fault during software testing phase, consists of both deterministic and incremental random parts, increases as the number of faults removed increases. Several software reliability and cost models by means of quasi-renewal processes are derived in which successive error-free times are independent and increasing by a fraction. The maximum likelihood estimates of parameters associated with these models are provided. Based on the valuable properties of quasi-renewal processes, the expected software testing and debugging cost, number of remaining faults in the software, and mean error-free time after testing are obtained. A class of related optimization problem is then contemplated and optimum testing policies incorporating both reliability and cost measures are investigated. Finally, numerical examples are presented through a set of real testing data to illustrate the models results.