A use case is a mechanism for recording the behavior of some system. It consists of a set of stakeholders with goals and shows the scenarios that may play out in completing those goals, the Main Success Scenario and Extenstions--alternate success scenarios and error cases. It provides a well defined method for producing requirements that fulfills several important criteria when properly written.

1. Clear concise description of behavior
2. Aids the requirements writer in capturing the extensions, via proper writing strategy as described by Alistair Cockburn in Writing Effective Use Cases
   
3. Assists developers in easily selecting the smallest possible units of behavior to implement - ie. a Main Success Scenario or an Extension
4. Provides a clear road map for developing system tests - a major failing of 1.1) shall do ... requirements lists
   

Guidelines for Writing use Cases

Reminders

Write something readable.

Work breadth-first, from lower precision to higher precision.

For each step:

For data descriptions (only put Precision Level 1 into the use case text):

The Writing Process

1. Name the system scope and boundaries.
   Track changes to this initial context diagram with the in/out list.
2. Brainstorm and list the primary actors.
   Find every human and non-human primary actor, over the life of the system.
3. Brainstorm and exhaustively list user goals for the system.
   The initial Actor-Goal List is now available.
4. Capture the outermost summary use cases to see who really cares.
   check for an outermost use case for each primary actor.
5. Reconsider and revise the summary use cases. Add, subtract, or merge goals.
   Double-check for time-based triggers and other events at the system boundary.
6. Select one use case to expand.
   Consider writing a narrative to learn the material.
7. Capture stakeholders and interests, preconditions and guarantees.
   The system will ensure the preconditions and guarantee the interests.
8. Write the main success scenario (MSS).
   Use 3 to 9 steps to meet all interests and guarantees.
9. Brainstorm and exhaustively list the extension conditions.
   Include all hat the system can detect and must handle.
10. Write the extension-handling steps.
    Each will end back in the MSS, at a separate success exit, or in failure.
11. Extract complex flows to sub use cases; merge trivial sub use cases.
    Extracting a sub use case is easy, but it adds cost to the project.
12. Readjust the set: add, subtract, merge, as needed.
    Check for readability, completeness, and meeting stakeholders' interests.

Pass/Fail Tests for Use Case Fields