We were recently asked the following question from one of our clients concerning the use of passive vs active voice when writing requirements:

“Is passive voice not allowed to state a requirement? Our engineers usually use passive voice to write requirements.”

Using passive voice when writing requirements is a very common defect we see in requirements.  A major problem with many specifications is that the requirement statements fail to state “who” is responsible and “what” must be done.  For example:  “28 VDC power shall be used by the system.”  This is passive in that the “who” is at the end of the sentence.  What makes matters worse, is when writing requirements in the passive form, the “who” at the end of the sentence is frequently dropped.  So now the requirement reads:  “28 VDC power shall be used.”   With this form you have to ask “By whom?” – it is not clear and is therefore ambiguous.  This can become a real problem when system requirements are mixed with people or statement of work requirements – does the requirement apply to the system or to the organization providing the system?  Does the requirement apply to the system as a whole or only part of the system?

Many organizations feel that the “who” is implied by the location of the requirement in the specification or the specification that the requirement is in.  Do not fall into this trap —  it can only lead to misunderstandings.  Write requirements in the active voice.  For this example:  “The system shall operate using 28 VDC power.”  Now it is clear who the requirement applies to – the entire system.

From an English grammar standpoint, requirements should always be written in the active voice using the subject(noun)-verb-object form. “Who” is responsible and “what” shall be done.

Use this simple active voice format:

• The system shall operate at a power level of xxxxxx,
• The software shall acquire data from xxxxxx,
• The structure shall withstand loads up to xxxxxx.

For the “who”, you want to control the nouns that you use…you don’t want to say “The GPS receiver shall xxxx.”, “The receiver shall xxxx”, “The unit shall xxxx” -avoid using synonyms.  Keep it simple.  Pick one name for your system of interest (SOI) and use it consistently for all requirements for that SOI. Good requirements are boring to read.  Use the same name (noun) for the system of interest for all requirements for that system.  If it is a door, say door, don’t change to gate, opening, portal, doorway, etc.  If you use synonyms, it can be confusing and the reader may think the requirement applies to a different system or part of the system.

Besides making the requirement easier to understand and unambiguous,  writing requirement in the active voice also helps determine which level and part of the system architecture the requirement applies to.  If the noun is “system” the requirement applies to the system as a whole.  If the noun is “software”, the requirement applies only to the software.   If the noun is “structure”, the requirement applies only to the structure.   If the noun is an organization, then the requirement is a statement of work requirement on a task or activity that organization must perform.

As with all our blog posts, your comments on this topic are welcome.

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