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The purpose of this article is to define the term software quality attributes and to place that term in the context of SQA and software process improvement, SPI.

ISO/IEC 9126 provides a framework for the evaluation of software quality.

It defines six software quality attributes, also called quality characteristics:

* Functionality: are the required functions available, including interoperabilithy and security
* Reliability: maturity, fault tolerance and recoverability
* Usability: how easy it is to understand, learn, operate the software system
* Efficiency: performance and resource behaviour
* Maintainability: how easy is it to modify the software
* Portability: can the software easily be transfered to another environment, including installability


McCall identified three main perspectives for characterizing the quality attributes of a software product.

These perspectives are:-

# Product revision (ability to change).
# Product transition (adaptability to new environments).
# Product operations (basic operational characteristics).

Product revision
The product revision perspective identifies quality factors that influence the ability to change the software product, these factors are:-
# Maintainability, the ability to find and fix a defect.
# Flexibility, the ability to make changes required as dictated by the business.
# Testability, the ability to Validate the software requirements.

Product transition
The product transition perspective identifies quality factors that influence the ability to adapt the software to new environments:-
# Portability, the ability to transfer the software from one environment to another.
# Reusability, the ease of using existing software components in a different context.
# Interoperability, the extent, or ease, to which software components work together.

Product operations
The product operations perspective identifies quality factors that influence the extent to which the software fulfils its specification:-
# Correctness, the functionality matches the specification.
# Reliability, the extent to which the system fails.
# Efficiency, system resource (including cpu, disk, memory, network) usage.
# Integrity, protection from unauthorized access.
# Usability, ease of use.

In total McCall identified the 11 quality factors broken down by the 3 perspectives, as listed above.
For each quality factor McCall defined one or more quality criteria (a way of measurement), in this way an overall quality assessment could be made of a given software product by evaluating the criteria for each factor.
For example the Maintainability quality factor would have criteria of simplicity, conciseness and modularity.

# Boehm identified seven quality factors, namely:-

# Portability, the extent to which the software will work under different computer configurations (i.e. operating systems, databases etc.).
# Reliability, the extent to which the software performs as required, i.e. the absence of defects.
# Efficiency, optimum use of system resources during correct execution.
# Usability, ease of use.
# Testability, ease of validation, that the software meets the requirements.
# Understandability, the extent to which the software is easily comprehended with regard to purpose and structure.
# Flexibility, the ease of changing the software to meet revised requirements.

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