It depends on your situation. If you look at the diagram in
Development Phases Examined you'll notice that there's no starting point. The implication is that you could, if you chose to, walk right up to a whiteboard and start doing data modeling (hopefully with at least one other person) without any other input.
Then again, perhaps your organization chooses to be incredibly inefficient and therefore requires a bunch of documentation to be written before you data model.
My advice is to choose to succeed.
- Scott
<a href="http://www-306.ibm.com/software/rational/bios/ambler.html" target="_blank" rel="nofollow">Scott W. Ambler</a><br />Practice Leader Agile Development, IBM Rational<br /> <br />Now available: <a href="http://www.ambysoft.com/books/refactoringDatabases.html" target="_blank" rel="nofollow">Refactoring Databases: Evolutionary Database Design</a>