Consider a requirement that says "The system must respond quickly to customer
enquiries". First we need to find a property of this requirement that provides
us with a scale for measurement within the context. Let's say that we agree that
we will measure the response using minutes. To find the quality measure we ask:
"under what circumstances would the system fail to meet this requirement?" The
stakeholders review the context of the system and decide that they would
consider it a failure if a customer has to wait longer than three minutes for a
response to his enquiry. Thus "three minutes" becomes the quality measure for
this requirement.
Any solution to the requirement is tested against the quality measure. If the
solution makes a customer wait for longer than three minutes then it does not
fit the requirement. So far so good: we have defined a quantifiable quality
measure. But specifying the quality measure is not always so straightforward.
What about requirements that do not have an obvious scale?