Software Metrics:

Software metrics can be classified into three categories:
1. Product metrics,
2. Process metrics, and
3. 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 in-process 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. Examples include:

Product quality metrics
• Mean time to failure
• Defect density
• Customer-reported problems
• Customer satisfaction

In-process quality metrics
• Phase-based defect removal pattern
• Defect removal effectiveness
• Defect density during formal machine testing
• Defect arrival pattern during formal machine testing

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.