# Misunderstanding about “Evidence Based Scheduling” in estimation tasks

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.

## 1 Answer

at the point 2, what total have 1% probability in every case? The total of example? But it has only 5 tasks.

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.

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"

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.