I read the Spolsky blog the post with title "Evidence based scheduling" (http://www.joelonsoftware.com/items/2007/10/26.html) but I don't understand the mechanism to predict and estimate interruptions.
If I understand, Splosky says to prepare 100 scenario for a task in ramdom way
At point 3 of the article, Spolsky says
1) "While calculating each possible future for a given developer, you’re going divide each task’s estimate by a randomly-selected velocity from that developer’s historical velocities, which we’ve been gathering"
2) "Do that 100 times; each total has 1% probability, and now you can figure out the probability that you will ship on any given date"
3) Spolsky says that, more the velocity of a developer has been regular in the past, more the differents values are convergent in the same value (as every Montecarlo simulation, I says too).
4) In next paragraph ("Obsessive-compulsive disorder not required") I tell us that there are a lot of different interruptions with differen amount of time (him example is about "Painful conversation about marlin"), but EBS don't receive any problem in precision of estimation.
Well, I don't understand why
- at the point 2, what total have 1% probability in every case? The total of example? But it has only 5 tasks.
- at the point 4, I don't understand the sentence "
is exactly the same as the probability that John’s boss would interrupt him during any given feature"
I think to lose same details, but I feel a bit confused.
What is my error?
PS I don't know if this question is more correct here or in Project Management Exchange: if I got wrong I'm sorry.
Best Answer
The estimation system takes 100 velocities calculated from the developers history. Each velocity is the ratio between estimated time and actual time for a given task.
It just means that because the interruption affects the velocity for a certain percentage of the tasks, the interruption will be appropriately represented in estimates that are based on 100 random velocities from the developers history.