title: What is the best way of formally expressing usability requirements?
body: <p>I am writing a system requirements document and need to include non-functional requirements relating to the usability of the system, but am not sure of the best way to express this.</p>
  
  <p>"The system shall be easy to use" seems a bit vague to me, and not testable. Are there any 'official' standards/guidelines that can be adhered to relating to the usability of a program?</p>