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When writing a specificiation, it is sometimes tempting to use non-testable text for normative requirements; indeed, making sure a text is testable requires much more work than using simple text without caring about testability (see also TestableOrNot).

But using non testable text as normative requirements has many drawbacks:

  • mandating something that cannot be tested is a no-op; how can you check whether something was indeed implemented if you cannot test it?
  • non-testable requirements means that implementations are likely to differ on the said requirement, meaning that interoperability will be loose at best
  • leaving a requirement in a fuzzy non testable state means leaving the disambiguation work to the implementors, making it much more costly and much more likely to generate confusion for the end users

See also a thread on www-qa.