Waterfall logic - cost of change again.

If

• (a medium-sized project with absolutely no change takes X days to complete) and
• (about 25% of requirements change on the average medium project - according to Boehm and others) and
• (lets say the average change, using Boehm's cost of change curve, takes 50 times the effort to change - it might be more, it might be less, depends when it happens)

then

• a medium-sized project with 25% change takes 13.5X days to complete [i.e. X + (.25 * 50) X = X + 12.5X]

Therefore, if we want the shortest project possible then we must prevent change up-front.  Even if it takes a bit longer to get it right.  If only that were possible.