5.5 Defect Removal Model

5.3 Quality

Quality is conformance to customer requirements. It is the number of customer requirements that are met and satisfied or a measure of how many customer requirements a software product or service satisfies. Conversely, poor quality is nonconformance to customer requirements or the number of customer requirements that have not been met or satisfied. Poor quality is a measure of how many customer requirements a product or service satisfies. For example, if all customer requirements have been satisfied by a software product or service, then the quality is 100%. If half of the customer requirements are satisfied by the software product or service, then quality is 50%. Defect density is another way of expressing the same principles. In other words, defect density is a measure of conformance to customer requirements. That is, the number of defects per lines of code is a measure of quality. In fact, defect density is by far the most popular measure of quality in the field of software engineering. However, we will examine a variation of defect density that expresses conformance to customer requirements a little better. Figure 15 illustrates the formula for quality.

Figure 15: Formula for Quality

How did we translate conformance to customer requirements to the application of defect density? Well, first of all defects are another way of saying that a requirement has not been satisfied. Also, defects per lines of code is a simple ratio of defects to software size. Defects refer to all classes of defects, including requirements, design, code, and test defects. Like productivity, defects are normalized using software size as a standard way of expressing quality. For instance, three defects per lines of code may mean one requirement, design, and code defect was found.

Quality measurement, like productivity measurement, is highly controversial for several reasons. First, there are the conscientious objectors. They think that quality, like beauty, is in the eye of the beholder and cannot or should not be measured. Then there are those who say quality is a measure of cost. That is, they reason that higher quality products cost more and lower quality products cost less. Some say that quality is a measure of customer satisfaction, customer happiness, and quality of service. Of course, there are those who say quality is a measure of ease of use, expressed as usability, user friendliness, or the use of graphical user interfaces versus textual ones. Finally, some say quality is a product or service from a well-known blue-chip corporation or is a well-known brand. The industry success of an organization or firm is quality to some.

Certainly all of these popular and perhaps even pervasive or ubiquitous definitions of quality are correct, and they can certainly be quantified , measured, and used for ROI of SPI. However, the vast number of perceptions will be reduced here to a measure of conformance to customer requirements. This is not an arbitrary decision, as conformance to customer requirements is a standard measure of quality. Quality is sometimes even referred to as a long list of attributes including maintainability, reliability, usability, modularity, etc. However, let's assume that failure to address any one of dozens of minor attributes counts as one defect and that defects are a nonconformance to customer requirements.