What is a task?
You will often hear agile teams talking about breaking product backlog items (PBI’s) down into tasks (or “sub-tasks”) but what is a task and what is the difference between a task and a product backlog item?
A useful way to think about tasks is to see them as the smallest unit of work that a team will undertake. A typical PBI (e.g. a user story) might be broken down into design, coding, testing and documentation tasks. Breaking work down in this way allows a cross-functional team to work on several tasks in parallel in order to complete the PBI collaboratively as a team.
Breaking down work into tasks also helps deliver PBI’s iteratively and incrementally with teams often aiming to deliver a completed PBI in 1-3 days. That being the case, tasks as smaller slices, can typically be delivered within hours.
What is the difference between a task and a PBI?
Several tasks would usually need to be completed to produce a finished PBI. A useful way to differentiate between a task and a PBI is perhaps to think of a PBI as describing some completed functionality that will be executed by a user when using a system, and a task as describing how the development team will complete the work – the what (PBI) and the how (the tasks).
So, a PBI would typically involve multiple types of work done by different people whilst a task would tend to be one type of work done by one person.
Why use tasks?
Tasks help to make progress visible, for example on a Kanban board. As it is the tasks that usually move along a Kanban board, they visually represent the flow of work through a system. This helps teams identify bottlenecks in their process and make visible any work that hasn’t yet been started so that the team can ask why that is.
Although the Scrum Guide doesn't specifically refer to tasks, it does say: "Work planned for the first days of the Sprint by the Development Team is decomposed, often to units of one day or less." Defining these small units of work as tasks and limiting them to a day or less can help developers create flow with focused, uninterrupted time.
Top tip
Agile teams will often create a set of common tasks, a template, for PBI’s that they then use to speed up task creation for each PBI. The template is used as a starting point with unnecessary tasks then removed as seen fit. A bit like applying a definition of done - not every item will apply to every story but it’s a good place to start.