At first, the only way to estimate is badly. Then, the only way to estimate well is to keep estimating.
If you listen to what different camps are saying about software development schedule estimation, you’ll find that much of the discussion can be viewed as riffs on the “agile” vs “traditional” divide. Undoubtedly, such discussion raises many instructive points about estimation techniques, but it frequently overlooks a basic truth about estimation:
Everybody is terrible at estimating the first few times around, no matter how they do it.
This principle is often misapplied as a condemnation of traditional estimation techniques. I frequently hear people dismissing traditional techniques, such as Gantt charts, because they found that some attempt to use the technique was largely unsuccessful. That’s a bit like dismissing the piano because you can’t play like Mozart the first time out.
Any estimation technique you employ will require practice. The first time you estimate a project of non-trivial duration, your estimates will likely be horrifically wrong. The same might go for the second and third times as well. However, sooner or later you’ll catch on to certain things (e. g., sign-offs always seem to take a week, Joe low-balls everything, Susan always forgets to budget time for her testing), and you’ll revise your estimates accordingly. Over time, the quality and fidelity of those estimates will undoubtedly improve.
So-called “agile” estimation techniques are smart enough to formulate this truth more explicitly, by emphasizing that you need multiple sprints consisting of the same team members in order to establish a reliable velocity. However, you’re not just collecting velocity data over those multiple sprints, as though you were dipping a thermometer in different parts of a pool in order to establish an average water temperature. You’re also honing the team’s ability to estimate things by giving them estimation practice coupled with near-immediate feedback on how good their estimates were. Over the course of those sprints, the team is getting their hands dirty in the nitty-gritty of estimation, and learning how to do it better each time out.
The punchline here is that somebody working with an agile method is likely to learn how to estimate faster/better because of short cycle times, but this doesn’t mean that traditional methods don’t work – it just takes longer to get good enough at using the traditional methods, and many folks give up after the first or second time out.
Much more important than any specific method of estimation is the determination to keep applying your chosen method until you become competent with it.