What a Design Specification Must Establish Before Any Concept Work Begins
A good specification does not choose the solution. It fixes what the solution must satisfy, within which limits, and how it will be judged. Concept work that starts without it is exploring in the dark.
The design specification matters as much as the requirements. The requirements govern it. How fully a specification can be satisfied depends on whether the requirements themselves can be fully satisfied. A specification built on requirements that cannot be met will fail, however well the design is executed.
What it is for
The specification is the design’s own statement of how the requirements will be met. It is settled before concept work begins, so that the concepts have something to be judged against.
Concept work that starts without it is exploring in the dark. Each concept can claim success against its own private idea of what the design should do.
What it fixes
A specification fixes four things above all: the targets, the design margins, the limits and the criteria. Alongside them sit the governing standards, the operating conditions and the interfaces.
Together these control the robustness of the outcome. They decide where the design is allowed to land, all the way through to the final design.
Clarity and verification
The specification must give clarity on the design parameters. It must also say how the design can be verified. It is the baseline that verification checks against.
Validation is separate. Validation checks the product against the need in real use. Keeping the two apart starts with a specification that is clear about which job it is doing.
Margin, codes and operability
Codes and standards provide the allowable design margin. The specification chooses a position inside that range.
It should not be narrowed for operability alone. A specification tightened around today’s operating convenience reduces repairability and applicability later. The product then struggles when its scope extends. A specification should be written for the full life of the product, not for the way it happens to be run on the first day.
What it leaves open
At concept stage the options stay open. The specification does not choose the solution. Each option is held to the requirements and drawn down to comparable specifications, with the same targets, margins, limits and criteria. This makes the comparison fair.
It also resolves a tension. A specification that fixes the criteria and leaves the solution open does not close off better options. A specification that quietly selects the solution does.
What drift does
A concept that drifts away from its specification does not simply disappear. Its consequences are carried into the implemented product. The operating window narrows. Extra counter features are added to make the design work.
Each one adds cost and complexity. None was part of the original need. This is the same cycle seen elsewhere, where patches add cost while capability erodes.
Defending the specification through the lifecycle
The concept has to defend its specification through the whole design lifecycle. The burden of proof sits with the concept. The specification is not bent to fit a concept that has moved.
The specification is also revisited as the design develops. The purpose is to confirm that the design output still meets the requirements.
If the output departs from the specification, there is a legitimate path. It is a deviation, with reasoning that demonstrates why it is valid, recorded at the time. Drift is different. Drift is gradual and unrecorded. A deviation is a reasoned decision.
Two kinds of deviation need to be kept apart. A deviation from the specification is a design decision. A deviation from a requirement needs the client’s agreement.
A specification is an ability
A specification is not a limitation. It describes the ability of the product to perform within its defined system.
A plain test shows whether it is ready. Could two concept teams work from it separately and be judged by the same criteria? If they could, the specification is doing its job.