Typically I have used the planning poker estimation sequence when performing task estimation (1,2,3,5,8,13). Here the numbers represent hours instead of story-points as they normally do during User-Story estimation. We used a cut-off of 13, the number of hours above which we think the task should be broken down. The basic idea behind it was that using the planning poker sequence for the number of hours a task would take, provided a buffer to the estimates.
Recently we had a discussion at work on whether tasks should be estimated in just plain hours as estimated by the developers, or whether we should use the planning poker sequence to drive the number of hours estimate.
The first thing to realize: its important to use different units for your user-story estimates vs. task estimates. This is accomplished by using story-points for user-stories and hours for estimates. This is useful because story-points are an abstract entity, making it easier to realize that the story-points are trying to capture not just effort, but also complexity and risk. In addition, it makes you also realize that story-points (as opposed to another unit like ideal hours) cannot be directly converted to number of hours that the user-story will take (although one can estimate this after a few sprints have passed and we have a team sprint velocity metric).
Back to the topic on hand: Based on research I have done, I am now a convert to using just plain hours for the task estimates (instead of using the planning poker sequence). Here is why:
- User stories are large, complex and have a lot of uncertainty and risk. They are harder to estimate. Hence we use the planning poker sequence to reiterate the fact that there is probably going to be a magnitude of difference in the amount of work/time it will take to complete the user-story.
- Tasks are how user-stories are broken down when it comes time to implement a user-story. An important point to remember is that a 3 hour task and an 5 hour task are not different by an order of magnitude, which is what the story-points reflect. Instead, the 2 tasks are different by 2 hours and that’s all there is to it. In addition, it is perfectly alright to have a task that would take 4 hours. By sprint planning stage, the risk and complexity associated with a user-story should be known. In addition, tasks are typically items that the developers know about and can estimate well. Finally, the number of tasks that need to be estimated are normally small because they are done for the current sprint (where the sprints typically last from 1 to 4 weeks – smaller the better). Because of the shorter time horizon on tasks, one shouldn’t have to pad the estimates for tasks. Hence, one shouldn’t have to use the planning poker sequence, and should just use the actual time estimate for that task.
Remember, based on the cone of uncertainty: Story points are useful for long-term estimation. Hours are better for short-term estimation.
Some other notes:
Task estimation should still be done by involving all the team-members. The user-story should be broken down into tasks and estimates should be presented by all team-members at the same time (just as one would do during a planning poker session). Any differences in estimates should be discussed and a consensus estimate in number of hours for the task should be arrived at. (This in my mind is a modified wide-band delphi estimation technique).
Normally, I have done sprint-planning in two separate sessions:
session 1: break down the user-story into tasks
session 2: generate estimates for the tasks.
Its important to also realize that an estimate is not the same as a commitment to deliver (i.e., an estimate of 8 hours for a task is not the same as committing to deliver the work in 8 hours). Hence the commitment session should be separated from the estimation session. The team commits to what it believes it can deliver within a sprint, given the estimates of all the tasks, other work that the team needs to do and other factors.
Its interesting to note that in the 2011 Scrum Update, the following is said (further validating the fact that estimates are not commitments):
Development Teams do not commit to completing the work planned during a Sprint Planning Meeting. The Development Team creates a forecast of work it believes will be done, but that forecast will change as more becomes known throughout the Sprint.
Comments by Geoffrey Corey (Agile mentor):
User stories are still abstract things and should be story pointed. Planning poker and relative estimating with a benchmark story are two ways to do that. The purpose for the story points is to prioritize and identify high risk or nebulous stories that either are broken down or are candidates for a spike solution. Once the stories are sized and prioritized then a time box (sprint) can be set and based on sprint velocity user stories can be added. At that point it is useful to break down just those stories in the sprint into tasks.
I totally agree that tasks should be done in hours, not story points. You can then add up the task hours and build a chart showing Story | Story Point | Total # of task hours. This will show you if your story point estimates are in line with task estimates. For example, you many have around 20 hours of tasks to do for a 5 point story. If you are within +/- 4 it appears you were consistent in story point estimates for all the 5 point stories. If you have on 5 pt story that is 60 hours of tasks, then you might want to pop the flare early before the sprint and inspect what went wrong and if you should repackage the sprint.
Velocity is still done by story points. Task estimation is just a validation of the planning poker estimate.
User story to task validation chart:
Total # of task hours
as a user I would like to pay using a credit card
as a user I would like to pay using a debit card
as a user I would like to register my account
as user I would like to login using Facebook Connect
as user I would like to reset my password
* the 3rd story probably has a bad story point estimate. the same is probably true for story 5.
- Task estimation: Do or don’t? http://www.agilejournal.com/articles/columns/column-articles/1232-task-estimation-do-or-dont
- Are you using planning poker for estimating tasks? Maybe you should reconsider it… http://geeks.ms/blogs/jlsoria/archive/2011/05/19/are-you-using-planning-poker-for-estimating-tasks-maybe-you-should-reconsider-it.aspx
- Sprint and release planning should be in different units: http://blog.mountaingoatsoftware.com/sprint-and-release-planning-should-be-in-different-units
- Why I don’t use Story Points for Sprint Planning: http://blog.mountaingoatsoftware.com/why-i-dont-use-story-points-for-sprint-planning
- Separate estimating from committing: http://blog.mountaingoatsoftware.com/separate-estimating-from-committing
- Agile estimation and the cone of uncertainty: http://agile101.net/2009/08/18/agile-estimation-and-the-cone-of-uncertainty/
- How to implement Scrum in 10 easy steps: http://www.allaboutagile.com/how-to-implement-scrum-in-10-easy-steps/
- Wideband Delphi: http://en.wikipedia.org/wiki/Wideband_delphi