Both Spoon and 37 Signals both get it right, it’s an uphill battle. It’s one thing for a customer/user to make uninformed implementation assumptions on a project, but managers do it as well. And not only do managers make the assumption, they may have promissed it informally to others, maybe even their bosses. So not only do you have to convince your boss that they underestimated, but that they have to tell their boss they underestimated. You are now battling on multiple fronts.

Of course, managers who are former developers, or at least those who have a firm technical understanding of the project are usually less inclined to make this mistake. On the other hand, this knowledge of the application and development in general makes their “guesses” feel all the more informed and accurate.

The good news is that acurate or inaccurate, the guess doesn’t change the amount of time it will take to complete the project. The bad news is that the guess does change the amount of time you have to deliver the project.