Why are design problems in equipment found only once it is on the floor?

Because, for equipment software, there has been nothing to check the design against until the machine runs.

• Unit testing in the ordinary sense is structurally impossible without the machine. The field has run on “if the machine moves, it’s fine” for decades — not because anyone was cutting corners.
• Verification therefore waits for integration and site commissioning.
• Requirements, system behaviour, abnormal handling and past design decisions are not expressed in an explicit form that can be checked. What is not expressed cannot be checked.

What it costs

• Days lost tracing the cause, a schedule that slips, parts rebuilt; at the worst end, a recall or a safety incident.
• No account line reads “design defect”, so the cost is felt but not measured.
• A long-standing rule of thumb in software engineering puts the cost to fix at roughly ×1 when found at design, ×10 at integration test, and ×100–1000 on site or in production. It is a rule of thumb, not a measurement of any particular company.

Human-readable versions: home · Why equipment software cannot run the right-hand side of the V-model

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