What is Quality?

The standard EN ISO 9000:2005, where fundamentals and definitions of quality management systems are described, defines quality as “degree to which a set of inherent characteristics fulfills requirements”.

One conclusion of this definition is that requirements must exist. Ideally requirements are agreed with the product owner and described clearly. Then these are explicit requirements. If requirements for specific product characteristics are not determined explicitely, they are not generally out of scope but called implicit requirements, i.e. typical expectations on these characteristics.

The next important conclusion is that the quality of a product can be determined by measuring the degree of fulfilling these requirements by product characteristics. This is easy in case of explicit requirements. For implicit requirements only the individual estimation of a tester can be used to decide whether the typical expectation of a product characteristics is fulfilled or not.

The recommendations for software development based on this view are:

• Requirements need to be described explicit and accurate; if possible, quantitative. Quantitative specifications can be verified precisely and should therefore be preferred over textual descriptions.
•  Implicit requirements must be avoided, because then the results of quality checks depend of the tester's individual estimations. They only can be avoided by describing them explicitely.