A completed (thorough and consistent) specification marks the end of research, and the beginning of development.
Prior to a completed specification, one is doing research (no matter how short or long this time may be). There are still unknowns. Unknowns require research. Research time is not predictable.
After a completed specification, one is doing development. The unknowns are hopefully minimal. Research is not required. Development time is predictable. An incomplete specification will allow research to slip into the development phase, thus making development time unpredictable.
