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Yassine Tounsi for Foretheta

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Why using t-shirt sizes to weight user stories doesn't help too much?

In order to address their needs, IT companies and startups adapt and customize their implementation of Scrum methodology. Processes have to be simpler with small teams and relatively smaller projects; however, some simplification practices may deprive product owners and project managers of the great advantages offered by Scrum. In this post, I will talk about the practice of using t-shirt sizes instead of story points to weigh user stories.

At a glance, both the t-shirt sizes and story points tend to appear indistinguishable. But when the practical aspects of it are considered, the difference is easily notable. To understand this, you need to analyze the practical aspects starting with the objectives of weighing stories.

What is the key metric in Scrum? The answer is only one word; Velocity. Velocity is the measure of the amount of work a team performs during a single sprint. It is calculated at the end of the sprint by totaling the points of all fully completed user stories. Imagine what can happen if we don’t have numbers given by story points!

When starting to evaluate user stories, comparing t-shirt sizes is not an easy approach. We cannot, for example, compare two user stories having S and L with one user story having XL or assume that XL is two times L. The statistics will be difficult to comprehend and used by both product owners and development teams. With story points, it’s quite simple; a story that is assigned 2 story points should be twice as much as a story that is assigned 1 story point. This makes story points more straight-word.

Let’s assume that all user stories are evaluated with t-shirt sizes. It will be almost impossible to define sprint velocity accurately. However, with story points, the product owner can easily find out what his/her team can tackle in a specific number of points over time. The objective is always to increase the number, but this will usually follow a linear curve.

For instance, if a team can perform 50 points per sprint, then it will be useless to plan a new sprint with 100 points. This is because planning a new sprint will result in an uncompleted sprint and overwhelming frustration. Planning 55 points or 60 points is the right and most appropriate decision. If well planned for, stories are done before the end of the sprint while additional stories are added from the backlog.

Project managers are most concerned when it comes to the team’s velocity. It is from this that they can control the project schedule and communicate reasonable deadlines to clients. This helps in decision making and outlines the importance of story points over t-shirt sizes.

Finally, we may all know how difficult and costly it is to change any process in the organization. The same applies when it comes to the decision to shift from using t-shirt sizes to story points. This is why there is a simple technique to use in the first stage by combining both of them. Here is an example of linking the Fibonacci sequence with T-Shirt sizes: 1XS, 2S, 3M, 5L, 8XL, etc. When everyone gets used to numbers, then t-shirt sizes could be removed forever.

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