One of the hardest aspects of Scrum seems to be the accurate costing of stories. We all know the theory: you break your work into chunks, cost those chunks, and commit to a certain amount of work each week based on costing and velocity. Pure Scrum™ then states that if you do not complete your committed work then Your Sprint Has Failed. All well and good, apart from one small problem: it’s all bollocks.
I’ve long had an issue with the traditional costing/commitment/sprint cycle insofar as it doesn’t translate from theory into the real world. In her recent talk at NorDev, Liz Keogh pointed out that Scrum practitioners are distancing themselves from the word “commitment” and from Scrum Failures as they are causing inherent problems in the process. Allan Kelly recently blogged about how he considers commitment to be harmful. Clearly I’m not alone in my concerns.
As always, when it comes to things like this, I’ve simply borrowed ideas for other people and customised the process to suit me and my team. If this means I’m doing it “wrong” then I make absolutely no apologies for it, it works for us and it works well.
Developers suck at costing. I mean really suck. It’s like some mental block and I have yet to find a quick and effective way to clear this. Story points are supposed to make developers better at costing because you’re removing the time element and trying to group issues by complexity. That’s lovely on paper, but I’ve found it just confuses developers – oh, we all get the “a Chihuahua is 1 point so a Labrador must be 5 or 8 points” thing, but that’s dogs, visualising the relative size and complexity of code is not quite as simple.
What’s worse is that story points only relate to time once you have a velocity. You can’t have velocity without points and developers generally find it really hard to guesstimate1 without times. Chicken and egg. In the end I just loosely correlated story points and time, making everyone happy. I’ve also made story points really short because once you go past a few days estimates start becoming pure guesses. What we end up with is:
|0||Quicker to do it than cost it.|
|0.5||One line change.|
|1||Easily done in an hour.|
|2||Will take a couple of hours.|
|3||A morning/afternoons work.|
|5||Will take most of the day.|
|8||It’s a days work.|
|13||Not confident I can do it in a day.|
|20||Couple of days work.|
|40||Going to take most of the week.|
|100||Going to take a couple of weeks.|
Notice this is a very loose correlation to time, and it gets looser the larger the story point count. Given these vagaries I will only allow 40 and 100 point costings to be given to bugs. Stories should be broken up into chunks of two days or less so you’ve got a good understanding of what you’re doing and how long it’s going to take2.
With that in mind 40 points really becomes “this is going to be a bitch to fix” and 100 points is saved for when the entire team looks at you blankly when diagnosing the problem: “Err… let me go look at it for a few days and I’ll get back to you“.
Story point inflation is a big problem with scrum. Developers naturally want to buy some contingency time and are tempted to pad estimates. Story point deflation is even worse with developers being hopelessly optimistic and then failing to deliver. Throw in the The Business trying to game the system and it’s quickly become a mess. I combat this in a few ways.
Firstly, points are loosely correlated to time. In ideal conditions a developer wants to be completing about 8 points a day. This is probably less once you take meetings, walkups and other distractions into account. While an 8 point story should be costed such as the developer can complete it in a normal day with distractions accounted for, the same doesn’t hold true for a series of 1 point stories. If they’re all about an hour long and there’s an hours worth of distractions in the day then the developer is only getting 7 points done in that day.
Minor fluctuations in average per developer throughput are fine, but when your velocity starts going further out of whack it’s time to speak to everyone and get them to think about how they’re estimating things.
Secondly, points are loosely correlated to time. A developer can track how long it takes them to complete an issue and if they’re consistently under or over estimating it becomes very apparent as the story points bear no correlation to the actual expended effort. A 5 pointer taking 7 hours isn’t a problem, but any more than that and it probably wanted to be an 8 pointer. Make a note, adjust future estimates accordingly. I encourage all my developers to track how long an issue really takes them and to see how that relates to the initial estimate.
Thirdly, costing is done as a group exercise (we play planning poker) and we work on the premise of an “average developer”. Obviously if we take someone who is unfamiliar with the code it’s going to take them longer. You’ll generally find there’s some outlying estimates with someone very familiar with that part of the code giving low estimates and people unfamiliar with it padding the value. We usually come to a consensus fairly quickly and, if we can’t I just take an average.
I am aware that this goes against what Traditional Scrum™ teaches us, but then I’m not practicing that, I’m practicing some mongrel Scrumban process that I made up as I went along.
Velocity and commitment
I use an the average velocity of the past 7 sprints3 adjusted to take into account holiday when planning a sprint. We then pile a load of work into the sprint based on this figure and get to work. Traditionally we’ve said that we’ve committed to that number of story points and issues but only because that’s the terminology that I learned with Scrum. Like everything else, it’s a guestimate. It’s what we hope to do, a line in the sand. There are no sprint failures. What there is is a discussion of why things were not completed and why actual velocity didn’t match expected velocity. Most of the time the reasons are benign and we can move on. If it turns out there are problems or impediments then we can address them. It’s a public discussion and people don’t get to hide.
Epics and Epic Points
The problem with having story points covering such a small time period is that larger pieces of work start costing huge numbers of points. A month is something like 200 points and a year becomes 2500 points. With only 2000 hours in a year we start getting a big disconnect between points and time which The Business will be all over. They’ll start arguing that if a 1 year project is 2500 points then why can’t we have 2500 1 point issues in that time?
To get round this issue we use epic points which are used to roughly cost epics because they’re broken down into stories and properly costed. While story points are educated guesstimates epic points are real finger in the air jobs. They follow the same sequence as story points, but they go up to 1000 (1, 2, 3, 5, 8, 13, 20, 40, 100, 250, 500, 1000). We provide a handy table that lets the business know that if you have an epic with
x many points and you lob
y developers at the problem then it will take roughly
z days/weeks/months. The figures are deliberately wooly and are used for prioritisation of epics, not delivery dates. We’re also very clear on the fact that if 1 developer can do it in 4 weeks, 2 developers can’t do it in 2. That’s more likely to be 3 weeks.
Epic points are malleable and get revisited regularly during the life of an epic. They can go up, down or remain unchanged based on how the team feel about the work that’s left. It’s only as the project nears completion that the epic points and remaining story points start bearing a relationship to each other. Prior to that epic points allow The Business to know if it’s a long way off, or getting closer.
1 What, you think this is based on some scientific method or something? Lovely idea, we’re making educated guesses.
2 I’ve had developers tell me they can’t cost an issue because they didn’t know what’s involved. If you don’t know what’s involved then how can you work on it? Calling it 20 points and hoping for the best isn’t going to work. Instead you need to create a costing task, and spend some time working out what’s involved. Then, when you understand the issue, you can then cost it properly.
3 A figure based purely on the fact that JIRA shows me the past 7 sprints in the velocity view.