ISO 2500 SQ quality definition goes as follows and I really do not understand the note with regards to ISO 9000:

capability of software product to satisfy stated and implied needs when used under specified conditions

Note 1 to entry: This definition differs from the ISO 9000:2005 quality definition mainly because the software quality definition refers to the satisfaction of stated and implied needs, while the ISO 9000 quality definition refers to the satisfaction of requirements.

I do not get it, because requirement itself is (in the same standard) defined as stated, generally implied or obligatory need or expectation. Therefore, if requirement is a need that might be implied, isnt both definitions the same? ISO 9000 reads the same except that instead of needs it reads requirements. But again, if requirements subsume needs (as stated in their definition), how these are different?

EDIT: I believe I got it - the main difference is that requirements are already translated needs that might have been misunderstood or captured incorrectly, while needs are in general abstract "wants" that users can assess while using the software. So physical products basically conform to certain set of requirements while software validation is more about perception and actual fitness for purpose from the user needs perspective.

  • I agree: the explanation in that quote is murky. ISO-9000 is only indirectly about quality; it is really about documenting your processes. See sqa.stackexchange.com/questions/17103/… for more.
    – user246
    Commented Mar 10, 2016 at 13:17
  • Actually when I think about it, I think I got it - while SW quality is mostly about satisftying needs (i.e. rather more abstract concepts), the product quality is more about requirements that are more specific and explicit. The key concept is that a requirement is a need that was translated to that form and might be misunderstood/distorted, which is very common. Validation of software by users is probably what they meant, while verification is just conformance to requirements. But it is really dodgy.
    – Pietross
    Commented Mar 10, 2016 at 14:48

1 Answer 1


The main difference it that requirements are what has been written down in the requirements documents, both the top level requirements document as supplied by the customer and in the derived requirements documents generated during the development process.

This does indeed differ from stated and implied needs in that there may well be implied needs that never got translated into requirements, i.e. never got written down because they were considered too obvious.

Consider the requirement "Given the input values of mass and height released from calculate the impact velocity at ground level allowing for atmospheric drag for a spherical object". This would probably have derived requirements specifying the formula(s) to be used. However, unless the people deriving the requirements were very thorough, there might not be a requirement for the code not to crash if a negative altitude or mass were entered but there would be an implied need or expectation.

So ISO 25000 SQ specifically states that implied needs should be tested while ISO 9000:2005 states that requirements must be tested and expects that such needs will make it into the requirements.

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