Story Estimation

We were having a chat internally at Bitnami with respect: should we try to standardise T-Shirt sizes across the team and also if we ought to have a mapping between user story points and implementation hours.

I felt that this is a discussion that keeps repeating, in conversations internally and externally, and I thought it would be worth it to put some effort into summarising my  thoughts about it.

Story Points vs Task Hours

Normally you will find Scrum coaches recommending the use of T-Shirt or Points for Story sizing, and hours for estimating Tasks. Stories are the unit of product/user value that it is delivered (tasks are just work).

Story sizing (T-Shirt or Points) is not an estimation of time, rather a way to express complexity. Yes, there is a correlation between complexity, effort and hours.  However, Complexity tends to be less variable across team members than hours.

A Story is complex due to the problems to be solved while implementing it, but the time it takes to solve the same problem might vary depending on the skills and experience of the person solving it.

OK, but how do we established consistency in estimating complexity? Human beings are pretty good at comparing similar problems. Problem A is similar to Problem B. So, I normally start by getting a team to agree the sizes of stories/problems that they have completed/solved together in the past. Then use that as reference point going forward.  This common understanding can only be shared by people in the same squad, since it is required a shared experience.

Story sizing can be and it is used to measure velocity, and also burn down progress. However, IMHO they are most useful to identify misalignment (Foo thinks is XXL but Bar thinks is L, how come? maybe Bar has some context that is missing Foo? or the other way around?). This conversation between team members about complexity is where the real value of Story sizing resides.

I think we can all agree that we can not predict the future.  Hence estimating in hours is always highly inaccurate. Inaccuracy grows as you extend the period of time you are estimating over (i.e. something that can take months or it is months away).

So why is it OK to estimate tasks in hours? Because tasks are small break down of stories that are assigned to an individual to implement.  Basically, the estimation is done over the shortest time span possible and taking the skills and experience of that person into account.

Tracking and managing cost

A different (and valuable) problem to solve is tracking the cost of implementing a feature/product/release for financial reasons. Most of us work on either For Profit or Cost Driven (non-for-profit) organisations, so having a record and an indication of where money (and hence hours) are going can be critical for our organisations.

While estimating time is highly inaccurate,  tracking actual time spent on tasks doesn’t have to be. Clearly there is a (human) cost associated on the granularity of this tracking. Which is the right level of tracking will be highly dependent on your business and accounting model.

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