Some quality models, like the probably best known one, ISO 25000 (SQuaRE project) defines quality model based on quality characteristics that are all non-functional. It explicitly reads that the model is not intended to evaluate functional aspects of software.

I get it - if something works as expected (e.g. a stapler) you cannot really decide about its quality until you assess non-functional aspects like its material, ergonomy, durability etc.

But at the same time, if the stapler does not work, no amount of non-functional requirements could make up for it.

In addition, the ISO standard defines quality as follows:

degree to which the software product satisfies stated and implied needs when used under specified conditions

So it is meant to be used in conjunction with functional assessment? IF the model itself does not deal with FR, how can I decide whether or not the software satisfies the needs?

1 Answer 1


To improve quality for a software product we are splitting up quality into internal and external quality. Spread over three quality groups. It gives a more detailed view on the different quality aspects that are relevant in software development.

External quality determines the fulfillment of stakeholder requirements and internal quality determines the ability to move forward on a project.

Functional Quality

  • Meeting the specified requirements.
  • Creating software that has few defects.
  • Good enough performance.
  • Ease of learning and ease of use.

Process Quality

  • Meeting delivery dates.
  • Meeting budgets.
  • A repeatable development process that reliably delivers quality software.

Structural Quality

  • Code testability.
  • Code maintainability.
  • Code understandability.
  • Code efficiency.
  • Code security.

Sources with more details:

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