To write a good requirement, you must write it as a complete sentence, with a subject and a predicate (usually a verb). The subject is an Actor, a stakeholder, the system under development, or a design entity that is related to the requirement. The predicate specifies a condition, action, or intended result that is done for, by, with, or to the subject.
Consistent use of the verb to be solidifies the link between the subject and the predicate. Thus, you can analyze a requirement from a grammatical point of view.
Beware of lists, bullets, and words such as all, every. and some. For example:
The order entry clerk must be able to complete 10 customer orders in less than two hours.
This requirement has a subject (the order entry clerk, who is an Actor), a specific and measurable end state (10 customer orders completed), and a performance criterion (in less than two hours).
Follow these simple guidelines in writing any requirement. For consistency, these examples are all in the context of an aircraft. [[WAS: is used throughout.]] [TEL06]
- Define one requirement at a time.
The pilot shall be able to control the aircraft's angle of climb with one hand.
The pilot shall be able to feel the angle of climb from the climb control.
- Avoid conjunctions (and, or) that make multiple requirements.
The navigator shall be able to view the aircraft's position relative to the route's radio beacons.
The navigator shall be able to view the aircraft's position as estimated by inertial guidance.
- Avoid let-out clauses or words that imply options or exceptions (unless, except, if necessary, but).
The design shall provide a rear-facing seat for each cabin crew member.
- Use simple, direct sentences.
The pilot shall be able to see the airspeed indicator.
- Use a limited (500-word) vocabulary, especially if your audience is international.
The airline shall be able to convert the aircraft from business to holiday charter use in less than 12 hours
Note: There is no need to use words such as reconfigured.
- Identify the type of user who needs each requirement.
The navigator shall be able to...
- Focus on stating what result you will provide for that type of user.
...view storm clouds by radar...
- Define verifiable criteria
...at least 100 miles ahead.