In building there are three stages: Preparation, Production and Proving.
Preparation. The environment must be prepared before work can commence. When painting a room, we have to choose the color scheme, measure, tape up the woodwork, and buy the paint, all before we can start putting it on the wall.
Production is the steepest slope where the maximum rate of measurable work occurs—the most code is written, the most paint is applied.
Proving is the final long tail of the process. This always seems to take longer than it should partly because we invariably find out things we were not expecting—which is where the Second Order Ignorance (unknown unknowns) comes in. In painting, this is the detail work, the tricky corners and, of course, the cleanup—which also always seems to take longer than it should.
All this means that when the product is 90% complete, the activity is only about halfway through its total time (so true).
Read more here (you will need access to CACM): How We Build Things