Here's how I like to think of it. Consider the project as a tree of goals with subgoals. The root goal is the whole project; it can be broken down into compon

Here's how I like to think of it. Consider the project as a tree of goals with subgoals. The root goal is the whole project; it can be broken down into components which are the subgoals; each of those has sub-components, all the way down to the individual lines of code. The fundamental problem is that until you're actually well into the project, you can only see down a few levels; but the amount of effort that is going to be required is a function of the length of the fringe of the tree, which you can't see, at least not very clearly, until you get there.