Christian Mackeprang has a new post to his site with some of his thoughts on why software estimates are impossible in any realistic project development process.

When you, as a programmer, start a new project, you will often not know full well how to do it, for many reasons. But you are a professional, and you’ve completed similar tasks in the past, so you either try to figure it out, or find someone who can, and ask them how, or just google it.

[...] The problem comes down to the difference between tasks which require a lot of thinking, and routine tasks which you already have some practice with.

He gives an example of solving a Rubik’s cube, how most people take a very long time to figure it out but there are some that can do it in a matter of seconds. He talks about unexpected complexity and how that can blow previous estimates out of the water. He points out that complexity can be cumulative (related to the number of tasks) and the difference between creative and mechanical tasks.