Heh, maybe that's because the nature of quality is qualitative rather than quantitative.

There are things that can be measured that might be correlated to quality ("bugs", sales, support, etc) but ultimately, classical quality seems to be to be a marriage of what was expected or desired with what was actually there. As a developer, my take is that quality usually stems from "developing" those expectations as much as making the relatively concrete thing to compare them too. When anyone can quantify the first half of this equation, please let me know at once.