Software metrics can be classified into three categories: product metrics, process metrics, and project metrics. Product metrics describe the characteristics of the product such as size, complexity, design features, performance, and quality level. Process metrics can be used to improve software development and maintenance. Examples include the effectiveness of defect removal during development, the pattern of testing defect arrival, and the response time of the fix process. Project metrics describe the project characteristics and execution. Examples include the number of software developers, the staffing pattern over the life cycle of the software, cost, schedule, and productivity. Some metrics belong to multiple categories. For example, the inprocess quality metrics of a project are both process metrics and project metrics.

Software quality metrics are a subset of software metrics that focus on the quality aspects of the product, process, and project. In general, software quality metrics are more closely associated with process and product metrics than with project metrics. Nonetheless, the project parameters such as the number of developers and their skill levels, the schedule, the size, and the organization structure certainly affect the quality of the product. Software quality metrics can be divided further into end-product quality metrics and in-process quality metrics. The essence of software quality engineering is to investigate the relationships among in-process metrics, project characteristics, and end-product quality, and, based on the findings, to engineer improvements in both process and product quality. Moreover, we should view quality from the entire software life-cycle perspective and, in this regard, we should include metrics that measure the quality level of the maintenance process as another category of software quality metrics.

Product Quality Metrics

  • Here are some of the Product related Quality Metrics
  • Mean time to failure
  • Defect density
  • Customer problems
  • Customer satisfaction.

In-Process Quality Metrics

Because our goal is to understand the programming process and to learn to engineer quality into the process, in-process quality metrics play an important role. In-process quality metrics are less formally defined than end-product metrics, and their practices vary greatly among software developers. On the one hand, in-process quality metrics simply means tracking defect arrival during formal machine testing for some organizations. On the other hand, some software organizations with well-established software metrics programs cover various parameters in each phase of the development cycle

  • Defect Density During Machine Testing
  • Defect Arrival Pattern During Machine Testing
  • Phase-Based Defect Removal Pattern
  • Defect Removal Effectiveness

Metrics for Software Maintenance

When development of a software product is complete and it is released to the market, it enters the maintenance phase of its life cycle. During this phase the defect arrivals by time interval and customer problem calls (which may or may not be defects) by time interval are the de facto metrics. However, the number of defect or problem arrivals is largely determined by the development process before the maintenance phase. Not much can be done to alter the quality of the product during this phase. Therefore, these two de facto metrics, although important, do not reflect the quality of software maintenance. What can be done during the maintenance phase is to fix the defects as soon as possible and with excellent fix quality. Such actions, although still not able to improve the defect rate of the product, can improve customer satisfaction to a large extent. The following metrics are therefore very important:

  • Fix backlog and backlog management index
  • Fix response time and fix responsiveness
  • Percent delinquent fixes
  • Fix quality
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