Bill Wake devised the acronym INVEST
as a reminder of the properties of good stories:
I - Independent
Stories ideally describe non-overlapping bits of functionality, possible to implement in any order.
N - Negotiable (and Negotiated)
Stories have flexibility; there's room for a collaborative definition of success.
V - Valuable
Stories' value makes sense from a customer or end user's perspective. They're not
described in terms of a developer's tasks.
E - Estimable
The team can estimate the difficulty of the story (at least to a rough level).
S - Small
The story is small enough for its purpose.
For implementation, it can be completed in less than an iteration.
For longer term planning, the story is larger.
T - Testable
The customer, developers, and testers can agree on what the story means precisely enough
that tests can be created.