(This article is part of the "Big Rewrite":http://chadfowler.com/2006/12/27/the-big-rewrite series.) In his article, The C2I2 Hypothesis, programmer Zed Shaw criticizes the famous C3 project at Chrysler, which is known for being the birthplace of eXtreme Programming. He says that the project was an implementation—not an invention. An invention, according to Zed, is something new which needs creativity and high customer involvement, whereas an implementation is a project which participants (including programmers) have done before. According to Zed:
If that's the case, why was the customer involved all the time? They had a completely working specification in an already working system. Replacing it is more a matter of reverse engineering than gathering vision, customer feedback, use cases, stories, or any of the other crap the XP team used.

Here’s the problem: when does the label "Payroll System" become so broad that you don’t know if it’s an invention or an implementation? Could it be possible that at a huge company like Chrysler the payroll system was unlike any other payroll system that had ever existed? And, within the realm of this possibility, might it also be possible that Chrysler were redesigning the system, because through such changes as globalization and evolving international tax and labor laws, the system which was being replaced was no longer valid?

My point isn’t to say Zed is wrong. He makes some excellent points and may very well be right (though, knowing some of the guys on the C3 project, I’d guess they knew the difference between invention and implementation).

My point is that it’s not always clear cut which things are implementation and which are invention. Worse than being ambiguous, it’s often not clear that it is ambiguous. My experience says that most of the time, people doing the Big Rewrite will assume that they’re doing an implementation and will staff and estimate accordingly.

Most of the time, they’re wrong.