What is software quality? What are McCall and ISO 9126 Quality factors?

To achieve a software of high quality is an ultimate goal. Software quality is assessed by internal and external quality criteria. External quality is critical to the user, while internal quality is meaningful to the developer only. Software quality is a mix of factors that vary across different applications and customers who request them.

McCall's Quality Factors
Software quality is affected by the factors that can be measured directly or indirectly. These include:
- During product operation, factors include correctness, reliability, usability, integrity and efficiency.
- During product transition, factors include portability, re-usability and inter-operability.
- During product revision, factors include maintainability, flexibility and test-ability.

ISO 9126 Quality Factors
This ISO was developed to identify quality attributes which includes:
- Functionality which includes the extent to which software is available for use indicated by sub attributes like suitability, accuracy, inter-operability, compliance and security.
- Reliability is amount of time software is available for use indicated by sub attributes like maturity, fault tolerance and recover-ability.
- Usability is the degree to which software is easy to use indicated by sub attributes like understand-ability, learn-ability and oper-ability.
- Efficiency is the extent to which software makes optimal use of resources indicated by sub attributes like time and resource behavior.
- Maintainability is an ease with which repair can be made to the software indicated by sub attributes like changeability, stability, testability.
- Portability is the ease with it can be transported from one environment to another indicated by sub attributes like adaptability, replace-ability.