6. Expression of needs and requirements

We indicated in the introduction that operational safety can be considered as a property of a system such that its users can place justified trust in the service they expect from it. However, these users must have a clear idea of the service they expect.

Ideally, future users should be able to express the "level of service" they expect from a system or piece of equipment, in order to verify that what a supplier is offering meets their requirements, or to enable designers to make the right choices to satisfy those requirements. These needs or requirements should be expressed in a functional "operational safety" specification.

• Except in cases where the operational safety approach is highly structured (aeronautics, nuclear power, railways, etc.), a few questions can help clarify the expected service or requirement to...