Design inputs usually fall into three categories:
• Functional requirements – what the system does
• Performance requirements – how the system does it
• Interface requirements – how the system interacts with the “world”
Four factors are in play when capturing these requirements: Clarity, Risk, Placeholders, and Approval.
1) Clarity - as various stakeholders are contributing to the design input stage, take care that terminology is well understood throughout. A term that means one thing to the groups who have conducted market research or risk analyses may have an entirely different meaning to a developer. For example, “the unit should have a rapid response” could mean anything from nanoseconds to “within 30 minutes”.
2) Risk - Different product subsystems must be evaluated for risk to ensure that the necessary level of necessary design detail is implemented. Hazards with higher severity and/or likelihood require more detailed design inputs.
3) Placeholders - At times, quantitative information is not available in this stage of the development. It is recommended that the requirement still be added, with a placeholder left for the numeric value to be filled in at a later date.
4) Once approved, control the design. Design inputs should only be changed through a formal design change process.
Once design inputs have been captured, they should undergo formal design review prior to approval. Qualified reviewers should ensure that design inputs are quantitative, measurable, and toleranced. For example, the following is a progression of requirements, from least to the highest level of specificity:
• The system should be rugged.
• The system should be able to handle being dropped from a table.
• The system should be able to handle a force of 1g.
• The system should be operational for 10 days following exposure to 1g.
• The system should be operational for at least 10 days following exposure to .95 – 1.15g.
Note that the final description of the requirement can be verified, while the original statement “should be rugged” is entirely unmeasurable.