It doesn't matter when you deliver, if you build the wrong product. Development entails inferences and assumptions about the user, which are supposed to guide the build-process. However, even if development successfully matches the inferences and assumptions about the user, if those criteria don't match the Real User, the product fails. This article talks about how to incorporate the user into the requirements and design phase.
The theory underlying a measurement must take into account at least nine factors. This article defines these nine factors (e.g., the scope of the measurement, the scale of the instrument, and the variation of measurements made with the instrument) and applies them to a few examples.