Epic vs user story
An epic is a body of work too large to finish in one iteration, split into several user stories as it nears the front of the backlog. A user story is one slice of that work, small enough for a single sprint and written from a user's view. Epic is common agile usage that the Scrum Guide does not define.
The longer version
Take a request to let customers export their data. As one item it will not fit in a sprint, so it is an epic. Splitting it yields stories that a user would notice one at a time: export a project as CSV, choose which columns to include, receive a large file by email. Each can be built, tested and released alone, and the epic is done when its stories are.
An epic carries a goal and a rough scope but few acceptance criteria. The criteria belong to the stories and are written as each one nears the front of the queue, since detail on work that is months away is mostly guesswork. Stories are the unit a team estimates in story points, and an epic's size firms up as its stories are sized.
Issue trackers tend to model the hierarchy as initiative, epic, story and sub-task, though the labels vary by tool and some teams skip a level or call the middle layer a feature. What matters is that the layer a sprint commits to is the story, so that is where the INVEST questions apply.
Common questions
What is the difference between an epic and a user story?
An epic is work too large for one iteration, and it is split into stories. A user story is a slice small enough to finish in one sprint, with its own acceptance criteria.
How do you split an epic into user stories?
Cut it into thin slices of working behaviour that a user would notice, such as one path through a flow at a time. Cutting by technical layer produces stories that nobody can use until all of them are done.
Is an epic part of Scrum?
The Scrum Guide does not define epics and speaks only of Product Backlog items. Epic is common agile and tool vocabulary, which teams use to group related stories under one goal.
Do epics have acceptance criteria?
An epic usually carries a goal and a rough scope, and the acceptance criteria sit on its stories. Some teams add a short statement of what must be true for the epic to close, which summarises the stories and does not replace their criteria.
How long can an epic stay open?
An epic stays open as long as its stories take to deliver, which can span many sprints or several releases. An epic open for a long time with few finished stories is often a theme or a wish, and it is worth splitting or closing.
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