I know this question has been asked several times on here/programmers and it is quite a common, classic question:
Just how do you accurately give estimates for how long a task will take?
The problem I have is for point-and-click tasks in Windows, I can give accurate estimates. For coding something new (as in using APIs I am not familiar with) I cannot estimate an accurate time to save my life. It's a case of thinking of and saying the first number (of days/weeks/whatever) that comes into my head. For code that uses APIs I am familiar with and apps I can instantly say I can develop (e.g. a notepad-type app) I could give an estimate that is accurate.
Any help appreciated.
Thanks
Focus on the pieces. When you try and estimate a task at a high level, not only is it daunting, but you will fail to accurately factor everything that will comprise the total time.
Instead, do not even try to guess at the total (not only is it not useful, but it can actually bias your estimates of individual tasks), but rather sit down and just try to think of all the sub tasks that comprise the task. If those feel too big, break them down into even smaller sub tasks.
Once you've done this, give each of the sub tasks an estimate. If any of them are larger then about 4 hours, the sub task probably hasn't been broken down enough. Add all of these sub estimates up. This is your estimate.
Using this approach forces you to reason out what is actually required to complete the task and will let you produce better estimates.
Make sure you think of the non obvious steps required to complete a task as well. If you're writing a piece of code, did you include time estimates for writing the associated unit tests? For testing the code? For documenting it?
When converting hours to days, use realistic expectations about how much time you actually spend heads down working. The general consensus is that a developer can complete 4-6 hours of work in any given 8 hour work day. This roughly matches my personal experience.
If you have other team members, you can try a technique called Planning Poker. At its simplest, the idea is to get each member of the team to go off and estimate each of the tasks individually. Once that is done, the team gets together and compares estimates looking for large deviations. This tends to bring to light if the task wasn't clear enough, if there are members of the team who posses relevant information that the others don't, or if there are different assumptions being made by different team members.
When doing your estimations, be aware of your assumptions and document them as part of the estimates. Assuming x, y, & x, task q should take n hours. These could be things like assuming the availability of a QA engineer to test the feature, assuming the availability of a development environment to deploy the feature to for testing, assuming the compatibility of two third party frameworks that haven't been tested together yet, assuming that feature z that the task is dependent on is ready by a certain date... Etc. We make tons of these assumptions all the time without being aware of them. Documenting them forces you to be aware of them and allows other parties to validate that your assumptions are correct. And if the estimates do turn out to be wrong, you have much more information to analyze why.
Even following all of this advice, you will still frequently make inaccurate estimates, but don't feel too bad because our brains are hardwired to produce terrible estimates for abstract tasks. We have a multitude of cognitive biases that affect our ability to gauge task size and effort.
I recommend reading this article for even more information and advice:
http://blog.muonlab.com/2012/04/12/why-you-suck-at-estimating-a-lesson-in-psychology/