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Patterns in the LSST:DM Sprint/Story-point/Story ‘done’ issues

Derek Jones from The Shape of Code

Projects that use Scrum as their project management framework estimate tasks (known as a user story, or just story) in units of Story-points. A collection of User stories are grouped together to be implemented during a Sprint (a time-boxed interval, often lasting 2-weeks).

What are Story-points, and how do they map to time (in hours and minutes)? For this post, let’s ignore these questions, simply assuming that the people who assign a story-point value to a story have some mapping in their head.

What is the average number of story-points in a story, and how does this average vary across teams? What is the distribution of number of stories estimated per sprint, how many are actually implemented, and how does this vary across teams?

The data required to answer these questions has not been publicly available, or rather public data is not known to me. Until this week, I had only known of a few public Jira repos where story-points were given for at most a few hundred stories.

The LSST Corporation, a not-for-profit involved in astronomy and physics research, has a Data Management (DM) project. The Jira repo for this project contains 26,671 ‘Done’ issues (as of Aug 2022), of which 11,082 (41.5%) have assigned story-points; there have been 469 sprints, which involved 33% of the issues. The start/end implementation date/time for stories is mostly rather granular, and not fine enough to be used to attempt to correlate individual stories with hours. I found this repo, and a couple of others, via the paper Story points changes in agile iterative development, and downloaded all available issues.

What patterns are present in the story-point and sprint data?

Story points are commonly thought of as being integer valued, but 28% of the values are non-integer. If any developers are using the Fibonacci scale, there are not enough to have a noticeable impact. The plot below shows the number of stories estimated to involve a given number of story-points (black pluses are non-integer values, which have been rounded to fit the regression model). The green curved line is a fitted biexponential (sum of two exponentials), with the two straight lines being the two component exponentials (code+data):

Number of stories estimated to involve a given number of story-points.

One exponential is dominant for stories assigned up to 10 story-points, and the second exponential for higher story-point values.

The development team decides to implement a story and allocates it to a sprint. A story may be reallocated to another sprint before the start of the original sprint, or after the sprint is finished when its implementation is incomplete or not yet started (the data does not allow for these cases to be distinguished). How many sprints is a story allocated to, before the story implementation is complete?

The plot below shows the number of stories allocated to a given number of sprints, with a fitted regression line of the form Stories approx Sprints^{-2.8} (code+data):

Number of stories assigned to a given number of distinct sprints.

So around 14% of stories are allocated to two sprints, 5% to three and 2% to four.

How many stories are assigned to a sprint? The plot below shows the number of sprints having a given number of stories assigned to them, and the number of sprints implementing a given number of stories; lines are fitted loess models (code+data):

Number of sprints assigned a given number of stories, and implementing a given number of stories.

Are the Story/Story-point/Sprint patterns found in the DM project likely to occur in other projects using Scrum?

I don’t know, but I hope so. Developing theories of software development processes requires that there be consistent patterns of behavior.

Not knowing what stories were assigned to a sprint at the start of the sprint, rather assigned earlier and then moved to another sprint, potentially undermines the sprint patterns. We will have to wait and see.

If anybody knows of any public Jira repos where a high percentage (say 40%) of the issues have been assigned story-points, please let me know (all the ones I know of on the Atlassian site contain a tiny percentage of story-points).

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NoEstimates panders to mismanagement and developer insecurity

Derek Jones from The Shape of Code

Why do so few software development teams regularly attempt to estimate the duration of the feature/task/functionality they are going to implement?

Developers hate giving estimates; estimating is very hard and estimates are often inaccurate (at a minimum making the estimator feel uncomfortable and worse when management treats an estimate as a quotation). The future is uncertain and estimating provides guidance.

Managers tell me that the fear of losing good developers dissuades them from requiring teams to make estimates. Developers have told them that they would leave a company that required them to regularly make estimates.

For most of the last 70 years, demand for software developers has outstripped supply. Consequently, management has to pay a lot more attention to the views of software developers than the views of those employed in most other roles (at least if they want to keep the good developers, i.e., those who will have no problem finding another job).

It is not difficult for developers to get a general idea of how their salary, working conditions and practices compares with other developers in their field/geographic region. They know that estimating is not a common practice, and unless the economy is in recession, finding a new job that does not require estimation could be straight forward.

Management’s demands for estimates has led to the creation of various methods for calculating proxy estimate values, none of which using time as the unit of measure, e.g., Function points and Story points. These methods break the requirements down into smaller units, and subcomponents from these units are used to calculate a value, e.g., the Function point calculation includes items such as number of user inputs and outputs, and number of files.

How accurate are these proxy values, compared to time estimates?

As always, software engineering data is sparse. One analysis of 149 projects found that Cost approx FunctionPoints^{0.75}, with the variance being similar to that found when time was estimated. An analysis of Function point calculation data found a high degree of consistency in the calculations made by different people (various Function point organizations have certification schemes that require some degree of proficiency to pass).

Managers don’t seem to be interested in comparing estimated Story points against estimated time, preferring instead to track the rate at which Story points are implemented, e.g., velocity, or burndown. There are tiny amounts of data comparing Story points with time and Function points.

The available evidence suggests a relationship connecting Function points to actual time, and that Function points have similar error bounds to time estimates; the lack of data means that Story points are currently just a source of technobabble and number porn for management power-points (send me Story point data to help change this situation).