Apart from being an oxymoron, there is a very simple reason why estimates cannot be “accurate” – we simply do not have the data necessary to be accurate. It is a sad fact that the earlier the estimate is made, the less data we have available, and therefore the less “accurate” we can be. The only time we have sufficient data to truly warrant the label “accurate” is at the very end of the project when all the variables have been resolved.

– Phillip Armour, The Business of Software, Communications of the ACM, November 2002, Vol. 45, No 11.