Marcel provides some insights on estimating. It is sound advice, except that estimation is usually done at a time when nothing else in the project is decided, which actually is guesstimation. So when we do this estimation, we define many other dimensions, like the team’s composition, skill-set and process. Unfortunately, a project rarely gets these resources as defined, and the estimation gets rarely re-calculated. Accuracy of an estimate can be valid for specific duration, among other moving factors. Estimation can be made more accurate only if it is made part of the feedback loop with the team, and regularly refreshed.