Types of Requirements

Types of Requirements

1. What are requirements ?

   • Behaviour or quality a new system is required to have
   • Behaviour requirements are usually called FUNCTIONAL
     • Something a system must DO
   • Quality requirements are usually called NON-FUNCTIONAL
     • Non-functional = how well the system must do something
       • How quickly, how easy to use, in what environment

2. Usability (System Quality for the user)

   • sub-class of non-functional, but so important they’re often listed separately
   • How easy, pleasant, error-free, stress-free the system is to use…
     • Users’ characteristics: screen must be accessible for visually-handicapped users
     • Tasks: interface must allow fast input of repetitious data without causing RSI(repetitive strain injury)
     • Situational factors: screens must be readable in low-light conditions

3. Functional (System Action)

   • What a system must do (description includes an active verb)
     • Enrol a student on a module
     • Create a new customer order
     • Produce monthly sales report
   • Will probably become programs or scripts in the final system
   • Initially documented by Use Case Diagrams
   • Can also be modelled with Data Flow Diagrams
   • Functional (Data) Requirements
     • Record all customer orders
     • Record all student enrolments
     • Modelled with Entity Relationship Diagram
     • Will become database in final system

4. Non-Functional (System Quality)

   • How, or how well the system performs
     • Response times
     • Volumes of data
     • Security issues
     • These will become measurable quality aspects of the final system

5. Why Specify Requirements?

   • Each system has unique requirements—need to be sure we get them right
   • Get this wrong, and we risk
     • Waste of money, time and resources
     • Disappointing users and sponsors
     • Lost business opportunities
     • Possible harm to people

6. Reasons to use Use Cases

   • Easy to understand
   • Easy to map onto business goals
   • Helps developers focus on building useful functionality
   • Helps divide a large project into smaller increments