5.7 Total Life Cycle Cost

5.5 Defect Removal Model

The defect removal model is a tool for managing software quality. It is a software project management method for software product quality. The defect removal model is a way for software project managers to plan, manage, and ensure the quality of their products. It is a tool that enables managers to plan, monitor, and optimize the application of appraisal activities. The Software Inspection Process, software testing, and automated static analysis are examples of appraisal activities. That is, on a phase-by-phase basis, software project managers will track the number of defects entering the phase. They will also track the number of defects injected during the phase and the number of defects exiting the phase. By doing this, they can establish software quality goals, design strategies to achieve the goals, and ultimately realize their goals. The defect removal model is a close cousin of defect removal efficiency, or, vice versa, defect removal efficiency is a close cousin of the defect removal model. The defect removal model yields the actual number of software defects that escape from a software life cycle phase or activity. Defect removal efficiency yields the percentage of defects removed by an activity. Figure 17 illustrates the formula for the defect removal model.

DRM = Defects Escaped + Defects Injected - Defects Removed

Figure 17: Formula for Defect Removal Model

Defects escaped is the number of software defects that exist prior to entering or performing a software activity or phase. Defects injected is the number of software defects committed during the phase or activity. Defects removed is the number of software defects identified and removed by a software appraisal activity. Let's say you enter the design phase and inherit 10 defects, create 90 architecture defects, and then remove 90 defects. The result of applying the defect removal model is 100 software defects less 90 software defects, or 10 software defects. The defect removal model is best applied on a phase-by-phase or activity-by-activity basis in order to optimize software quality. That is, use the defect removal model to track the number of defects that are being entered, injected, and removed. The defects are being removed from the requirements, design, code, and tests on a phase-by-phase basis.

The defect removal model is a tool for software project managers. The defect removal model is a way for software project managers to take charge of planning, managing, and optimizing quality. Gone are the days when software quality was a job for software test analysts and software quality assurance analysts. Software quality is the job of the software project manager. Software quality is just as important to managers as planning, managing, and optimizing cost and schedule performance. Software quality management using the defect removal model may be more important than cost and schedule performance. Why? Because cost and schedule performance are incontrovertibly linked to software quality performance. In other words, software project managers cannot achieve their cost and schedule targets without achieving their quality targets. If you don't achieve your software quality targets, both cost and schedule will be exceeded dramatically.

It is important to note that the defect removal model is only a tool. It does not have the precision and validity to be used for estimating software quality and reliability to any degree of accuracy, but it remains an important tool for enabling higher quality and reliability. Other forms of statistical quality and reliability models should be used for accurately estimating software quality and reliability. However, dollar for dollar, the defect removal model is by far one of the simplest, most useful, and easiest to understand tools. The defect removal model can be a persuasively effective method for optimizing software quality. Finally, the defect removal model is a critical source of key benefits for the fields of SPI and ROI of SPI.