Scrum: story points, ideal man days, real man weeks

My team completed its seventh sprint of a project. Once again all stories were accepted by the product owner.

While one of the team member was giving the sprint demo, I started looking in more detail at some of the numbers. Because with seven sprints behind us, we've gathered quite some data on the progress from sprint to sprint. That's the velocity, for XP practitioners.

Looking at the data

So far we've had 131 story points of functionality accepted by the product owner, so that's an average of 18-19 per sprint. The distribution has not really been all that stable though. Here's a chart showing the number of accepted story points per sprint:


Although it is a bit difficult to see the trend in sprint 1 to 5, we seemed to be going slightly upward. This is in line with what you'd expect in any project: as the team gets more used to the project and to each other, performance increases a bit.

The jump from sprint 5 to sprint 6 however is very clearly visible. This jump should come as no surprise when I tell you that our team was expanded from 3 developers to 5 developers in sprint 6 and 7. And as you can clearly see, those additional developers were contributing to the team velocity right from the start.

But how much does each developer contribute? To see that we divide the number of accepted story points per sprint by the number of developers in that sprint:

Apparently we've been pretty consistently been implementing 5 story points per developer per sprint. There was a slight drop in sprint 6, which is also fairly typical when you add more developers to a project. But overall you can say that our velocity per developer has been pretty stable.

Given this stability it suddenly becomes a simple (but still interesting) exercise to try and project when the project will be completed. All you need in addition to the data from the previous sprints, is an indication of the total estimate of all stories on the product backlog. We've been keeping track of that number too, so plotting both the work completed vs. the total scope gives the following chart:

So it looks like we indeed will be finished with the project after one more sprint. That is of course, if the product owner doesn't all of a sudden change the scope. Or we find out that our initial estimates for the remaining stories were way off. After all: it's an agile project, so anything can happen.

Story points vs. ideal man days vs. real man weeks

Whenever I talk about this "number of story points per developer per sprint" to people on other projects, they inevitably ask the same question. What is a story point? The correct Scrum answer would be that it doesn't matter what unit it is. It's a story point and we do about five story points per developer per sprint.

But of course there is a different unit behind the story points. When our team estimates its stories, we ask ourselves the question: if I were locked into a room with no phone or other disturbances and a perfect development setup, after how many days would I have this story completed? So a story point is a so-called "ideal man day".

From the results so far we can see that apparently this is a pretty stable way to estimate the work required. And stability is most important, way more important than for example absolute correctness.

A classic project manager might take the estimate of the team (in ideal man days) and divide that by 5 to get to the ideal man weeks. Then divide by the number of people in the team to get to the number of weeks it should take the team to complete the work. And of course they'll add some time to the plan for "overhead", being the benevolent leaders that they are. This will give them a "realistic" deadline for the project. A deadline that somehow is never made, much to the surprise and outrage of the classic project manager.

I'm just a Scrum master on the project. So I don't set deadlines. And I don't get to be outraged when we don't make the deadline. All I can do is study the numbers and see what it tells me. And what it tells me for the current project is that the number are pretty stable. And that's the way I like it.

But there is a bit more you can do with the numbers. If you know that the developers in the team estimate in "ideal man days", you can also determine how many ideal man days fit into a real week. For that you need to know the length of a sprint.

Our team has settled on a sprint length of four weeks. That's the end-to-end time between the sprints. So four weeks after the end of sprint 3, we are at the end of sprint 4. In those four weeks, we have two "slack days". One of those is for the acceptance test and demo. The other is for the retro and planning of the next sprint.

So there is two days of overhead per sprint. But there is a lot more overhead during the sprint, so in calculations that span multiple sprints I tend to think of those two days as part of the sprint.

So a sprint is simply four weeks. And in a sprint a developer on average completes 5 story points, which is just another way of saying 5 ideal man days. So in a real week there is 1.25 ideal man days!

I just hope that our managers don't read this post. Because their initial reaction will be: "What? What are you doing the rest of the time? Is there any way we can improve this number? Can't you people just work harder?"

Like I said before: I don't believe in that logic. It's classic utilization-focused project management. It suggests that you should try to have perfect estimates and account for all variables so that you can come to a guaranteed delivery date. The problem with that is that it doesn't work! If there's anything that decades of software engineering management should have taught us, is that there are too many unknown factors to get any kind of certainty on the deadline. So until we get more control of those variables, I'd much rather have a stable velocity than a high utilization.

Scrum: utilization vs. velocity

At work we're recently started using Scrum for running some projects. As expected we need to slowly learn the lessons. One of the things we're been having a lot of discussion on recently is the meaning of the focus factor. Let me begin by explaining what a focus factor is, at least in my company.

To determine how much work you can do in a sprint, you need to estimate the top stories. We estimate these stories in "ideal man days" using planning poker. This means that each developer answers the question: if we lock you into a room each day without any distractions, after how many days would you have this story finished?

After these estimates we determine people's availability for the project. After all, they might also be assigned to other projects, if only for part of their time. Even people that have no other projects, tend to have other activities. Like answering questions from customer support or consultants, department meetings, company wide meetings, job interviews with candidates or just playing a game of fusball, table tennis or bowling on the Wii. So basically nobody is available to a project 100% of the time. At most it's 80% - 90% and on overage it seems to be about 60% - 70%.

So the first stab at determining how much work someone can complete is:

• available hours = contract hours * availability
  

But when you're working on the project, you're not going to always be contributing towards the goals that you've picked up. Within Scrum there is the daily Scrum meeting. It lasts no more than 15 minutes, but those are minutes that nobody in the team is working towards the goal. And after the meeting a few team members always stick around to discuss some problem further. Such time is very well spent, but it probably wasn't included in the original estimate. So it doesn't bring the "remaining hours" down very much. I see all this meeting, discussion, coaching and tutoring as necessary work. But work that doesn't bring the team much closer to the goal of the sprint. I used to call this overhead, but that sounded like we were generating waste. So in lieu of the agile world I switched to using the term focus factor. So now we have:

• velocity = contract hours * availability * focus factor

So the speed at which we get things done (velocity) is the time we're working minus the time we loose to non-project work minus the time we loose on work that doesn't immediately get us closer to the goal. In the past I probably would have included a few more factors in there, but in an agile world this is already accurate enough to get a decent indication of how long it will take us to get something done.

If there's one thing I've learned from the agile movement and Scrum it's to focus on "when will it be done" instead of "how much time will it take". So to focus on velocity instead of utilization.

Utilization is the territory of classic project management. It's trying to make sure that every hour of every employee is fully accounted for. So if they're programming, they should have a time-writing slot for programming; if they're meeting, there's a slot for meeting; if they're reviewing designs, there's a slot for that and if they're drinking coffee or playing the Wii... you get the picture. Of course there's no project manager that wants all that level of detail. But in general they are focused on what you're spending your time on.

Agile thinkers see this really differently. They say: it doesn't really matter how much time you spend, what matters is when it is done. This sounds contradictory so let's see if a small example can make it clearer what I'm trying to say.

If I tell my boss that some feature he wants will be done at the end of next week, he is interested in only one thing: that it is done next week. If we get it done on time, he doesn't care whether I spent two hours per day on it or whether it was twelve hours per day. I care about it of course, because I don't want to work late every night. And there's also a limit to the amount of gaming I like to do during a day, so two hours per day will leave me bored quickly. But to my boss, all that matters is when I deliver, not how much effort it took.

This is why the focus for Scrum projects is on velocity and not on utilization. So in Scrum you want to know how many hours you still need to spend on a job, not how many you've already spent on it. A classic project manager might be really proud that you worked late all week and clocked in 50+ hours. An agile project manager will note that you reduced the "hours remaining" by 10 hours and nothing more. If you're looking for compliments on all your hard work, then Scrum might not be for you.

Learn more:

• Scrum: story points, ideal man days, real man week


• a simple tool to generate burndown charts online


• Who estimates the stories in Scrum?