Thursday, 2 September 2010

When Points Become Hours

I was recently working on a Story Pointing exercise for one of my customers who is running a Scrum based project using User Stories (for those not familiar with User Stories, Scrum and Story Pointing, please take a look here and here). This exercise took the form of a Planning Poker session (for info on Planning Poker look here). This all works great in theory, but this project had fallen into one particular trap.....
The notion of a Story Point is that it is supposed to be a measure of story size and complexity relative to other stories in the same backlog. So, if I start with a story that I estimate as 2 points then a story that seems twice as big and complex would be 5 points (on a scale 1, 2, 3, 5, 8, 13, 20, 40, 100), while a story half the size and complexity would be 1 point. It shouldn’t be necessary to know exactly how the story will be implemented in order to gauge its size and complexity relative to other stories.
However, this particular project had over time reached a stage whereby a Story Point had become associated with a particular number of hours development and test effort. This is a dangerous situation to get in to, but very easy to achieve - especially on long running projects like this one where a large amount of historic data had been collected supporting this association.
Two specific problems that occur when Story Points become measures of number of hours of implementation effort are:
Solutionising During Story Pointing
People by nature are generally much more comfortable thinking in terms of tasks and how long they will take as opposed to relative size and complexity. Therefore, when there is a link between the two they tend to switch to the more comfortable approach of asking “how long would it take me to do this piece of work?”. Once they have this figured out they then convert this value back to Story Points.
The problem with this is that to answer the ‘how long’ question you have to know what the solution will be. Therefore you have to work out how you would implement the story then break it down into tasks and then estimate these tasks - usually all done in your head in the space of a few minutes. Chances are some tasks will be missed - meaning the estimate is wrong. Also likely is that when a team later comes to implement the story they will pick a different solution (or the best solution might have changed) and therefore come up with a completely different estimate. 
Any Story Points derived in this way are therefore meaningless unless the people estimating the story immediately go out an implement it using the exact solution used during the pointing process.
Unwillingness to Commit
The other problem with linking Story Points and implementation hours is that people then feel that giving a Story Point estimate is a commitment to complete the story in the associated number of hours. If people feel that this commitment will be enforced then they either become unwilling to commit to a Story Point estimate until they have every last detail nailed down - hence nothing gets sized or sizing takes forever. Alternatively they inflate their point estimates by a significant (usually random) amount to account for uncertainty and unknowns and thus the sizes are no longer meaningful. 
So, when a project gets into this situation, how does it get out of it? Unfortunately, the only real way to overcome the association between Story Points and hours is to repoint the entire backlog. Start with a medium size/complexity story that is pretty well understood (but which has not been implemented or estimated) and  pick an arbitrary size somewhere in the middle of the point range chart (say 5 or 8). This ensures there is no starting link between hours and size. Then estimate all other stories relative to this starting story. This could be a major undertaking for a project with a huge backlog - but it's essential in order to move forward.
And, how do you avoid getting into the same situation again? That is fortunately easier: don’t track hours against stories. A Story Point is a measure of size relative to other stories. Hours are a way of measuring the progress of a team within a sprint. There’s no link between the two and trying to put one in place is always a bad idea.

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