Missing limits

IBM® Engineering Requirements Quality Assistant (RQA) message: This requirement has a value with missing or unclear limits. Effective requirements state specific boundaries so that results are easier to measure and verify.

Explanation

Always specify a range for each quantitative condition or exception in a requirement. It’s important to do so because physical systems rarely meet a quantitative target with 100 percent accuracy. Even the high precision instruments work with a small degree of error. When the range is specified, the permissible error tolerance is defined.

In requirements, when you specify quantitative condition, make sure that you provide the required details, such as unit, tolerance, and operator.
  • UNIT: Unit of measure (Examples: m, sec, v, or cm and expanded SI units, such as meter, second, volt, or centimeter)
  • TOLERANCE: Numerical value
  • OPERATOR: Sets a specific range for TOLERANCE (Examples: greater than, larger than, no less than, between x and y, from – to, and within)
If you did not provide any value for TOLERANCE, RQA might identify a missing limit from the following terms that are used in the requirements:
  • More than
  • Less than
  • Within
  • Above
  • Below
  • Between
  • Greater than
  • Less than
  • Higher than
  • Lower than
  • Under
  • Over
  • Fewer than
  • +/-

Example 1

  • The speed of the toy car in water should be 90 km/h.

Specify the limits so that results are easier to measure and verify. To eliminate the missing limits issue, you can make the following modifications:

  • The speed of the toy car in water should be 90 km/h with +/- 0.05% tolerance.

In exceptional cases, don't specify tolerance ranges for requirements with small countable values of discrete items. For example, a car should have five passenger seats, each with a seat belt.

Examples
  • The motor shall operate at 10 Nm between 15 mph and 18 mph.
  • The engine shall have a torque of 10 Nm for 25 seconds when the speed is between 15 mph and 18 mph.
  • The system will respond within 750ms.
  • The system must notify a customer that his payment is overdue after 3 days and before 5 days.
  • The point of sale system must take between 2 hours and 4 hours to process a request.