Project stories, defined by the product owner, will follow Bill Wake's INVEST model where "INVEST" stands for:
- Independent
- Negotiable
- Valuable
- Estimable
- Small
- Testable
Each of the stories in a project should follow the above six criteria before they are put onto the Product backlog. Stories should be independent, meaning that they should never overlap with another story and stories should be negotiable, meaning that the story when created will be a collaboration. Stories also need to be valuable to the product customer (not just to those creating the software). You should be able to estimate how long the story will take to implement and how large the story will be. Stories will be small - each story should only involve a couple of weeks work for one person. Finally, and perhaps most important, the story should be testable. If not, then there could be a huge problem in implementing the story during the Sprint.