Characteristics of good requirements – unambiguous

A requirement that is unambiguously documented can be understood in only one way [IREB]. While (or when) writing down a requirement avoid ambiguous words like e.g. “support”, “adequate”, “approximately, “complete”, “survive”, “handles”, “fault tolerant”, “user friendly”, “as much as possible”, “robust”, “several”, “as fast as possible” because  they mean something different to everyone who reads them.

Why do we need an unambiguous requirement?

Ambiguous requirements are subject to multiple interpretations, are not objectively verifiable, and create an opportunity for the requestor to ask for additional work without additional financial/time compensation.

Example:

The unit shall survive exposure to the non-operating temperature range of -40C to +65C.

What does the word “survive” means in this example. Readers can interpret that after the exposure the unit shall be functional. Readers can ask a question if the unit has to operate during this exposure as well as afterwards. It would be better to phrase the requirement as follows.

In the switch off mode the unit shall withstand the exposure to the non-operating temperature range of -40C to +65C.

After the exposure to the non-operating temperature range of -40C to +65C the unit shall perform at the same level as before the exposure.

Tips:

  1. Use terms consistently – define them in a glossary or data dictionary.
  2. Use a limited vocabulary – a simple subset of English, avoiding terms that may confuse non-technical or foreign readers.
  3. Avoid let-out clauses or words that imply options or exceptions (unless, except, if necessary, but, etc.).
  4. Focus on stating what result the requirement will provide/do for a stakeholder.

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