Let's start with a warm up exercise back from school.
We have 4 voltmeters as in the picture below.
What is the total voltage if the voltmeters read:
V1 = 1V, V2 = 1V, V3 = 2V and V4 = 3V ?
You say 7V? Yeah, sure, maybe in your high school physics class, but definitely not in engineering lab.
What I mean is of course margin of error during measurement (please excuse my poor vocabulary).
I haven't given the margin explicitly, so you should have used the industry standard 0.5 of the meter's unit (which obviously is 1V, as otherwise I would say 1.0V. You should have guessed that too, of course)
So the answer is 7V±2 : most probably 7, but maybe 5 or 9.
Lets come back to software engineering.
I have 4 tasks, I estimate the first will take me 1 day, second too, third 2 days and the last one 3 days.
High school student may say the whole project will take me 7 days. Engineering student knows it will be from 5 to 9.
But hey, we are experienced software developers, so we know that developer's estimate has at best 30% error margin. If we are product managers we know we should multiply the developer's estimate by 8.
We are not product managers, so lets assume 30% (rounding up to half days, because we all know what happens when developer finishes the task an hour before lunch)
Real task estimates: T1,2 = 1d±0.5, T3 = 2d±1, T4 = 3d±1
Project estimate (on Monday): 7d±3, which means somewhere between this Thursday and the following weekend (unless I catch a cold, realize that task 4 is much bigger then I thought or Christmas happens)
To make things more interesting, do you remember how to calculate error margin for division?
(Oh yes, I definitely have 4 developers to divide work among, that flue that Bob has caught from Charlie is definitely not long term)
What does it all have to do with agile project management?
Well, if you commit only to 2-week iterations, you can be no more then 2 weeks behind the schedule before the project manager starts noticing.
Compare that to 6 month tasks that are 80% done until the last day, when they begin to be 90% done for next 6 months.
Coming soon: why Auto adjust in JIRA's Log Work action sucks :-)