[sumnole]

Scope is sometimes fixed too, ie through contractual obligations. Velocity can be increased by pulling more resources in - to a certain point.

[lushdogg]

Agreed. Identifying what is fixed and what is negotiable is key. Most often I see that scope is the only one that is not fixed when it comes down to it.

Even more important is not letting your team get boxed into fixed scope and fixed time as you can increase velocity some but increases to velocity are usually the most over-estimated in my experience.