6. Expressing needs and requirements

In the introduction, we indicated that dependability can be considered as the property of a system that enables its users to place justified confidence in the service they expect from it. However, these users must have a clear idea of the expected service.

Ideally, the future user should be able to express the "level of service" he expects from a system or piece of equipment, in order to check that what a supplier offers matches his requirements, or to enable designers to make the appropriate choices to satisfy his requirements. These needs or requirements should be expressed in functional specifications.

• With the exception of cases where the operational safety approach is highly structured (aeronautics, nuclear power, railways, etc.), a few questions can be used to specify the expression of the expected service or...