Empirical and face validity of software maintenance defect models used at the jet propulsion laboratory

Context: At the Mission Design and Navigation Software Group at the Jet Propulsion Laboratory we make use of finite exponential based defect models to aid in maintenance planning and management for our widely used critical systems. However a number of pragmatic issues arise when applying defect models for a post-release system in continuous use. These include: how to utilize information from problem reports rather than testing to drive defect discovery and removal effort, practical model calibration, and alignment of model assumptions with our environment. Goal: To show how we can develop confidence in the practical applicability of our models for obtaining stable maintenance funding. Method: We describe the strong empirical and face validity we have investigated for our maintenance defect discovery and introduction models. We discuss the practical details of calibration and application within a functioning maintenance environment. Results: We find that our models, despite their simplicity, appear quite valid. Conclusions: The models are useful in justifying and obtaining stable maintenance funding.