When someone asks me for an estimate I like to pose this question: You just realized that you lost your keys. How long will it take you to find them? You've lo

When someone asks me for an estimate I like to pose this question: You just realized that you lost your keys. How long will it take you to find them? You've lost your keys before so you should be able to estimate how long it will take, right? Well, maybe. Probably you can retrace your steps and figure out where you set them down while you were on the phone, or maybe they fell into the couch. There is also a possibility that it is going to take you a long time to find those keys. The software estimate often depends on environments being available, other people finishing the piece you interact with on time and working the way that you expected, and you may have tricky defects that you really can't estimate how long it will take to fix them or how many there will be. A clear specification is only one piece of the puzzle. To switch metaphors, pretend it is 1812 and try to give an estimate of how long it will take to get from Chicago to San Francisco. Yes, you have a map with routes, but there is also weather, broken wagon wheels, dysentery, bandits, you name it. (just ask the Donner party). Let's just estimate it at 90 days and probably we'll lose some people along the way and pick up a few more. At best I try to give the suitcase estimate: I can estimate that you will need one suitcase for a weekend trip, you most likely will not need two.