In the book, "How To Run Successful Projects III, The Silver Bullet", Fergus O'Connell uses a scoring system to predict project success.

1. (20) Visualize the goal
2. (20) Make a list of jobs
3. (10) There must be one leader
4. (10) Assign people to jobs
5. (10) Manage expectations, allow a margin of error, have a fallback position
6. (10) Use an appropriate leadership style
7. (10) Know what's going on
8. (10) Tell people what's going on
9. (00) Repeat steps 1-8 until step 10
10. (00) The Prize

What this means is that having clarity on what you want to accomplish and being able to identify the work to be done (steps 1 and 2) are the more significant indicators of project success. 

After managing several projects over the years, I tend to agree.  Step 2 is particularly interesting because it not only helps you calculate schedule and budget, but it helps you identify the right people for the jobs.