Steele Bit programming & development musings by Andrew Steele

Story cards and use cases are both mechanisms for documenting user goals. Story cards are part of the agile software methodology Extreme Programming (XP). Use cases are part of the more formal Unified Process (UP) methodology. The two approaches share similarities but differ in their application. It is these differences that make one method more appropriate than the other depending on the type of project.

Story cards are user goals written in the form of two to three sentence stories on index cards. A story card contains enough information for the customer and developer to understand the story’s intent and estimate its implementation difficulty. In contrast, there are three use case “dressing” levels. A “briefly dressed” use case does not differ greatly from a story card in the amount of information documented. A “casually dressed” use case captures additional information but is not as detailed as a “fully dressed” use case-- a highly detailed document including pre/post conditions, alternative paths, and exception handling.

While story cards and briefly dressed use cases resemble each other in the amount of information documented, their function differs. Briefly dressed use cases are essentially placeholders for future fully dressed use cases. Fully dressed use cases are the final result of many customer and developer implementation conversations. While not every use case must be fully dressed before development begins, generally no development work towards implementing a particular use case begins until that use case is fully dressed. Conversely, story cards act as a conversation starter allowing programmers to begin development while still engaging the customer for implementation details. Establishment of user acceptance criteria for the story cards occurs during these conversations. From these criteria developers are able to create automated user acceptance tests. Automated user acceptance tests are a code form of the formal written documentation in a fully dressed use case.

XP story cards enable concurrent conversations during development, while UP use cases enable preemptive conversations prior to development. The difference between preemptive or concurrent conversations goes to the heart of the differences between agile and formal development methodologies. Agile methodologies and XP in particular attempt to engage the customer throughout a project’s development lifecycle. Story cards are a means to continuously engage the customer while developing a product. Formal methodologies like UP primarily focus on detailed documentation before implementation work begins. A fully dressed use case attempts to detail all possible usage outcomes creating a blueprint for a developer to work from.

Whether story cards or use cases are more appropriate for a particular project depends on whether an agile or formal methodology is most appropriate for the project. Projects that are life critical, require a very high degree of precision, or the development team is highly distributed will benefit from the use of fully dressed use cases and a formal methodology. Projects that allow for repetitive iterations, have a low fault tolerance, and lend themselves to close developer/stakeholder collaboration will benefit from the use of story cards and an agile methodology.

Much like agile and formal methodologies, story cards and use cases have advantages based on the type of project under development. Developers should be aware that while they may prefer one methodology over another, project requirements may determine which methodology is more appropriate.