Right-Size Your User Stories
A user story is a form of lightweight requirement that Agile projects use instead of long formal use cases. Use cases in all their detail don't adapt easily to changing customer needs. Instead, a user story provides just enough information to start a conversation between a developer and product owner. It's also the smallest piece of functionality that would provide value to the end user. Examples (from Mike Cohn's Advantages of User Stories for requirements) :
- A user can post her resume to the web site.
- A user can search for jobs.
- A company can post new job openings.
- A user can limit who can see her résumé.
Using Bill Wake's mnemonic, we INVEST in good stories: They're Independent, Negotiable, Valuable, Estimable, Small and Testable.
The trick, as Patrick says, is to know how much detail to write and when to write it. Too much detail early on and, like use cases, the story will have to be rewritten many times before it's implemented. Too little detail and the developers don't know what to plan for or implement. Patrick:
For stories that need to be implemented now, you want to have enough precision that allows developers and testers to be clear about what needs to be achieved. The waste of not having enough detail here is essentially rework in many of the downstream activities.
… For stories that need to be implemented in the distant future, you don't need the same level of detail. The waste of capturing too much detail too early is essentially rework at the analysis level.
So the answer is it depends: the further away a story is the less detail it should have. Only stories that are about to be tackled should have test cases and related details.