There are four types of Uncertainty in Projects
Variation - comes from many small influences and yields a range of values on a particular activity
Foreseen Uncertainty - are identifiable and understood influences that the team cannot be sure will occur
Unforeseen Uncertainty - can’t be identified during project planning
Chaos - projects subject to unforeseen uncertainty start out with reasonably stable assumptions and goals, projects subject to chaos do not. Even the basic structure of the project plan is uncertain, as is the case when technology is in upheaval or when research, not development, is the main goal.
These ideas are from the work of Arnoud De Meyer, Christoph H. Loch and Michael T. Pich in the MIT Sloan Management Review, WINTER 2002. Their work can also be found at www.insead.edu.