I like this analogy, and it applies to a number of projects I've been involved with. On the other hand, at work I also encounter change requests that are basica

I like this analogy, and it applies to a number of projects I've been involved with. On the other hand, at work I also encounter change requests that are basically repeats of existing projects with slight modifications, and/or changes to well understood subsystems such that I can pretty much nail the estimate. Not surprisingly, these are also relatively simple changes. It's the whole "known unknowns" vs. "unknown unknowns" thing, and I think it's useful to distinguish between the two.