On Estimates

There is a lot of room for miscommunication about estimates, as people have a startling tendency to think wishfully that the sentence:

I estimate that, if I really understand the problem, it is about 50% likely that we will be done in five weeks (if no one bothers us during that time).

really means:

I promise to have it all done five weeks from now.

from How to be a Programmer, by Robert L. Read.