For most software development organizations, ensuring quality means dealing with defects. Three generic ways to deal with defects include:

1) Defect Prevention
2) Defect Detection and removal
3) Defect Containment.

Defect prevention through error blocking or error source removal: These QA activities prevent certain types of faults from being injected into the software. Since errors are the missing or incorrect human actions that lead to the injection of faults into software systems, we can directly correct or block these actions, or remove the underlying causes for them. Therefore, defect prevention can be done in two generic ways:

- Eliminating certain error sources, such as eliminating ambiguities or correcting human misconceptions, which are the root causes for the errors.

- Faultprevention or blocking by directly correcting or blocking these missing or incorrect human actions. This group of techniques breaks the causal relation between error sources and faults through the use of certain tools and technologies, enforcement of certain process and product standards, etc.

Defect reduction through fault detection and removal: These QA alternatives detect and remove certain faults once they have been injected into the software systems. In fact, most traditional QA activities fall into this category. For example,

- Inspection directly detects and removes faults from the software code, design,
- Testing removes faults based on related failure observations during program execution etc.

Various other means, based on either static analyses or observations of dynamic executions, can be applied to reduce the number of faults in a software system.

We use defect detection and removal for the overall concept and activities related to what many people commonly call “debugging”.

When specific activities related to “debugging” are involved, we point the specifics out using more precisely defined terms, including,

- Specific activities related to defect discovery, including testing, inspection, etc.
- Specific follow-up activities after defect discovery, including defect diagnosis, analysis, fixing, and re-verification.

Defect containment through failure prevention and containment: These containment measures focus on the failures by either containing them to local areas so that there are no global failures observable to users, or limiting the damage caused by software system failures. Therefore, defect containment can be done in two generic ways:

- Some QA alternatives, such as the use of fault-tolerance techniques, break the causal relation between faults and failures so that local faults will not cause global failures, thus “tolerating” these local faults.

- A related extension to fault-tolerance is containment measures to avoid catastrophic consequences, such as death, personal injury, and severe property or environmental damages, in case of failures. For example, failure containment for real-time control software used in nuclear reactors may include concrete walls to encircle and contain radioactive material in case of reactor melt-down due to software failures, in order to prevent damage to environment and people’s health.

Various QA alternatives and related techniques can be used in a concerted effort to effectively and efficiently deal with defects and assure software quality. In the process of dealing with defects, various direct defect measurements and other indirect quality measurements (used as quality indicators) might be taken, often forming a multi-dimensional measurement space referred to as quality profile. These measurement results need to be analyzed using various models to provide quality assessment and feedback to the overall software development process.

By extension, quality engineering can also be viewed as defect management. In addition to the execution of the planned QA activities, quality engineering also includes:

- quality planning before specific QA activities are carried out,
- measurement, analysis, and feedback to monitor and control the QA activities

In this respect, much of quality planning can be viewed as estimation and planning for anticipated defects. Much of the feedback is provided in terms of various defect related quality assessments and predictions.

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