The biggest problem with estimation is the amount of information we assume. We make assumptions on how to solve the business problem, the technologies we're going to use, and the capabilities of the people building it. So many factors.
The level of complexity and uncertainty impacts our ability to give an accurate estimate because there are so many variables at play. This, in turn, is amplified by the size of the piece of work. The result is something referred to as the Cone of Uncertainty:
Barry Boehm first described this concept in his book Software Engineering Economics, 1981; he called it the Funnel Curve. ...
