In a dream world a developer should code all the day, right? Well, a very common task beside coding though is to estimate tasks of manifold kinds. Estimating though, sucks. For badly educated product managers and for many of our customers a estimate equals to a personal commitment. Therefore you better get it right the first time!
In many projects at itemis we use something we call WBS which stands for work breakdown structure. The term itself is not new, but rather PMI vocabulary. But the combination with three-point-estimates and some statistics makes it a very powerful tool.
Work Breakdown Structure
WBS is not more than breaking your tasks down to small units that can easily be estimated. It’s just a estimation technique and can be applied in any traditional or agile process.
Three-point Estimates
Three-point estimates help you to incorporate the risk into your estimate. Lets say I discovered a simple task, by breaking down the big project of writing a particular blog entry about WBS.
Example Task: Graphically illustrate the WBS domain model
Now think about how you potentially would solve this. The simplest solution would be to create a diagram of the classes I already have in Visual Studio. That would be almost no work. But those look to cheap and they contain the wrong details. So I’ll do it manually. Probably using OmniGraffle – a diagramming tool for Mac only.
Now, ask yourself three questions. How many hours would it take to finish this task, if…
- … I’m highly motivated, in the middle of a diagramming rush, and no tools crash, no one calls me, nobody asks for help on some other task, …? You see where this goes. Since there is a great stencil available that I’ve used earlier, I’d just drag the classes on the canvas and the auto-layout will probably be good enough. Lets say, in the best case I’ll need about 1 hour to illustrate the domain model.
- … I’ve done this before, and I know, visualizing is always tricky. Based on my experience I’d expect a little bit of trouble for the layout. It will probably also be necessary to adjust the stencil a little bit. Also I’d ask some of my colleagues for their opinion and then do some adjustments. In the average case I should be done in 4 hours.
- … I’ve had a busy week, it’s Monday morning 9 am and I am absolutely not in the mood to work at all let alone do anything creative. OmniGraffle crashes all the time, and the stencil turns out to not do the job. The telephone rings all the time, and my room mate happens to have a loud discussion with his PM because he just spent 3 days on a task he estimated with 2 hours. Also I have to start from scratch once because I forgot to save, and another time because I just messed up the layout. This is not really likely to happen all at once, but in the worst case I’d be on the tasks for 12 hours. I couldn’t imagine how it could take longer.
Now I’ve got three numbers. In the best case 1 hour, at average 4 and in the worst case 12 hours.
Statistics
Now, you can neither tell your customer nor your PM that you need one and up to 12 hours to finish the task. For one single task this might even be correct, but when you have a set of many tasks it’s not very likely that the worst case applies for every task.
I’m not really into the details of the statistics, but I’ll try to explain them from what I’ve understood.
There is two interesting values that can be calculated per task. Let us just name
- The pert is an educated guess. It considers the average case (A) for times more reliable than best (B) and worst case.
The pert formula: (B + 4A + W)/6 - The variance is a value that indicates how different best and worst case are. There is a magic number involved assuming that in 50% of the cases, the actual hours will less or equal to the pert. This is based on a suggestion from the book Software Estimation by Steve McConnell, page 121 ff.
The variance formula: (W–B / 1.4)^2
Applying these values to our task, we will result in following table. I also added another simple task
Task | Best | Average | Worst | Pert | Variance |
Graphically illustrate the WBS domain model |
1 |
4 |
12 |
4.83 |
61.73 |
Embed the domain model illustration in the post* |
0.08 |
0.25 |
0.50 |
0,26 |
0.09 |
Sums |
1.08 |
4.25 |
12.5 |
5.10 |
61.82 |
Standard Deviation** |
7.86 |
* A small task with a small variance has little impact. As you would expect. But if the variance grows, even small tasks have great impact. As in reality.
** The standard deviation is calculated by powering the sum of all variances with 0.5.
The first interesting result is, that by a certainty of 50% I’d manage to fulfill the two tasks within 5.1 hours.
Based on the standard deviation combined with the normal distribution (Gaussian curve) we can create a table indicating by which certainty (in %) we are done within X days.
Since the variance in our case is higher than the expected value, some of the numbers won’t make sense.
Each of the percentages have a factor of the variance with which the result deviates from the pert value.
Formula: Pert + (Variance * Variance Factor)
by a certainty of |
variance factor |
done within (hours) |
10% | -1.28 | (negative number) |
16% | -1 | (negative number) |
20% | 0.84 | (negative number) |
25% | 0.67 | (negative number) |
30% | 0.52 | 1.01 |
40% | 0.25 | 3.13 |
50% | 0 | 5.10 |
60% | 0.25 | 7.06 |
70% | 0.52 | 9.19 |
75% | 0.67 | 10.37 |
80% | 0.84 | 11.70 |
84% | 1 | 12.96 |
90% | 1.28 | 15.16 |
98% | 2 | 20.82 |
Result
No I can assure, that the task will be done within 20 hours by a certainty of 98%.
This number is just to high, isn’t it? But did it never happen to you that a task took 4 times longer than expected? Not so uncommon, I’d say.
Maybe I wouldn’t put this number on the customer offer, but that is not what it is about either.
What it is about is knowing you risk and make educated decisions.
But not yet enough.
Team estimates
All those calculations still rely on one person doing expert estimates. So what about having multiple persons that estimate the tasks and then combining the results?
The concept is simple.
- Most likely two persons do the breakdown together. Either one does a first draft and the other reviews it, or what I rather prefer is that they pair on this step.
- The two or more do the estimations separately.
Why separately? Well, it is not about blaming people but about gathering quality data. And having a high variance between to peoples estimates on a certain task indicates a high level of uncertainty about what this task means in practice. The earlier you identify those spots, the better.
Just get the two to talk and eliminate the uncertainty. They can then either agree on a estimate, or what at least should happen they redo the estimates with a lower variance. Both results raise the quality of your data.
There is different ways of combining multiple estimates. The one that makes the most sense is taking the lowest best case, the average of the average cases and the highest worst case. This would give you a quite reliable result.
Checklists
Especially meant to improve the quality of the worst case estimates the estimating person is guided by a checklist about typical things to consider when estimating software projects.
The current ones we use at itemis now include around 50 entries four categories: Preconditions, Things typically forgotten, Non-functional requirements and Other crosscutting concerns
The Experiment codenamed “Lillesand”
“Lillesand” is beautiful a city in the south of Norway. I just spent my Christmas holidays there. But for this project it’s just a codename.
Currently we use a Excel sheet that helps us creating the WBS. This works fine. But its limited. It lacks support for gathering multiple WBS in a project and the calculations for team estimates are still done manually.
Since this is much about data and functions, I thought I’d try to build this using the technologies leveraged by SQL Server Modeling formerly called Microsoft Codename “Oslo”.
I already posted on the migration to the latest CTP. I’ll publish the sources soon.
I don’t really know where this will go, but I think it will be interesting to follow.
“…………… certainty of 50% I’d manage to fulfill the two tasks within 5.1 hours.”
I haven’t understand how you calculated certainty value is 50%, I think certainty value should 61.82 ?
Thanks for writing nice Article on estimation.