Functional and Non-Functional Requirements

Functional Requirements

Something that the system must do (Robertson & Robertson 1999) – the most common type of requirement

Example: The web-site system shall purchase a request ticket

Functional requirements must be measurable and testable.

Non-Functional Requirements

They express the quality of the system and there are different types:

  • performance requirement – time to do things
  • look-and-feel requirement – how end user perceive the product
  • device requirement – features of the product
  • accuracy requirement – level of precision to be achieved
  • usability requirement – how people will interact with the product
  • training requirement – level and nature of training to use the product
  • availability requirement
  • maintainability requirement
  • recoverability requirement
  • portability requirement
  • reliability requirement
  • security requirement
  • safety requirement



Reference: Neil Maiden (2011) Requirements Engineering Lecture Notes

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