Use-Cases.

Writing Effective Use-Cases, A. Cockburn, Addison Wesley 2001
Applying Use-Cases (2nd ed), G. Schneider and J. Winters, Addison Wesley, 2001
www.rational.com has a couple white papers on use-cases on line.

Use-Cases are scenarios about how external actors interact with the system.
Anything outside the control of the system, a person or another system,
   is a actor.
Use-Cases do not expose interfaces between internal components in the system,
   There are other (Work-Flow) methodologies for those specifications.
They are used as part of defining the "requirements" for a project.
Use-Cases evolve over time, adding more details as the project evolves.
They also help in working out:
   Formal validation for the system.
   The Testing plan.
   Acceptance Criteria.
There are CASE tools for specification of Use-Cases visually. Rational 
   (Rose} and UML have adopted Use-Cases as part of their formal 
   system development process.

Use-Case Template: (from Pressman)

Use-case:        <use-case name>, Entering a new party.
Primary Actor:   <actor name>, Sales Manager.
Goal in context: Entering a new party into the data base.
Preconditions:   All information about the party is known.
Trigger:         A new party has been arranged.
Scenario:         <steps the actor takes>
                 1) Manager starts program
                 2) Manager enters password
                 3) Manager selects "new party"
                 4) Manager enters data for new party
                 5) Manager saves party
                 5) Manager exits program
Exceptions:
                 1) program is already running, data base is open ...
                 2) party name already exits
                 2) password wrong
                 3) date fields wrong
                 4) Manager exits program with out a SAVE or CANCEL
Priorities:
                 Essential, must be implemented
When Available:
                 First Prototype Version
Frequency of Use:
                 1 time a week (assuming one party a week is scheduled)
Channel to Actor:
                 Via Control Pannel Interface
Secondary Actors:
                 Support technician
Channels to Secondary Actors:
                 Via Internet
Open Issues:
                 ...

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Sample Data sets? 
How do you know that Use-Case was handled properly?