People in traditional projects attempt to define and then control scope during the project, while Agile teams approach the problem differently: figure out enough to get started, embrace changes, and then proceed as the way opens. Agile teams, by definition, try to respond to changing market or user needs as soon as those needs are discovered, yet business requires them to plan ahead from time to time. Agile teams know that they cannot predict a project's outcome, so they use pragmatic planning approaches to tune their development efforts to the latest and greatest needs. This is somewhat like what your local meteorologist does every night on the six o'clock news—his next-day forecast is usually spot on, but the seventh day is always off! Unfortunately, no team (or weatherman) can predict the future, which is why I keep a backup umbrella in my car.
The traditional project metrics of all planned scope, on budget, and by the deadline don...