An analysis of defect densities found during software inspections

Software inspection is a technical evaluation process for finding and removing defects in requirements, design, code, and tests. The Jet Propulsion Laboratory (JPL), California Institute of Technology, tailored Fagan's original process of software inspections to conform to its software development environment in 1987. Detailed data collected from 203 inspections during the first three years of experience at JPL included averages of staff time expended, pages covered, major and minor defects found, and inspection team size. The data were tested for homogeneity. Randomized samples belonging to the various phases or treatments were analyzed using the completely randomized block design analysis of variance ( α = 0.05). The results showed a significantly higher density of defects during requirements inspections. The number of defect densities decreased exponentially as the work products approached the coding phase because defects were fixed when detected and did not migrate to subsequent phases. This resulted in a relatively flat profile for cost to fix. Increasing the pace of the inspection meeting decreased the density of defects found. This relationship held for major and minor defect densities, although it was more pronounced for minor defects.