Another aspect of the design that can change rapidly and often is the testing and diagnostic approach itself. As changes are made to the system, due to improvement for maintenance purposes, reduction of critical behavior, elimination or control of feedback loops, etc., the testing approach typically changes in response.Since assessment hinges on an accurate representation of how the system will be diagnosed, it is critical to be able to rapidly configure various testing approaches. In fact, the Testability or Systems Engineer will often try out different testing scenarios to kick start the optimization process.
eXpress approaches this problem by allows flexible topological abstraction as a core of its testing approach. Engineers can define test coverage using high-level definitions that relate to the topology. This not only abstracts the topology, making it stable against topological changes, but also reduces the time usually spent manually determining fault propagation for the purpose of calculating test coverage. Going further, eXpress supports hybrid definitions--that is, both functions and failure modes can form parts of the definition. This means that early functional testing can give way to later failure mode testing, without destroying traceability or convergence towards the ultimate solution.
Rapid Diagnostic Assessment, whose foundation is accurate test definitions, is another essential factor in determining scalability.