Data-driven design is a vital tool for hill-climbing iteration of a site, but you should take great care not to use it as an appeals process whenever you and your designer reach an impasse.
via blog.fawny.org
This quote reminds me of an idea that is close to my heart -- the best engineers are designers and the best designers are engineers. You can skip all this madness if both designer and engineer are on the same page (OR are the same person) and aren't fighting over stupid things using various argumentative methods.