Quality(T) = Changeability(T) / Known defects(T)

What can we say about software quality and defects?

  • Software quality is inversely proportion to the number of defects in the system: high quality implies few defects and vice versa
  • Defects have undesirable consequences
  • Defects incur costs, in all likelihood financial costs but there are others, time in particular. Even if defects are not fixed they will incur costs, e.g. over payments from a financial system or people ringing the helpline to report a spelling mistake
  • Removing defects requires rework and rework costs time and money

define “Easy to change”

  • Change is localized
  • Change is less error-prone – perhaps better stated as “change does not inject defect” (Somewhere in Jones writing he suggests 7% of defect fixes inject new defects so high quality software would have a bad fix injection rate less than 7%)
  • Change is more maintainable, i.e. changing software does not detract from the changeability of the software