The determination of the field of study is a very important stage for the it’s success: a too important field will lead to a poor result; moreover it will limit the depth of the study, i.e. the level of detail to which it will be carried out. In any event, it is necessary to define it in order to be able to study it within two to three months, while knowing that the people constituting the work group will have to invest a lot of time. Functional cutting will make it possible to treat the study more quickly. Of course, this field limitation will depend on the aim of the study, the time available to the members of the work group, their knowledge of the criticality, the system failures or the similar systems, as well as the knowledge of the environment. These choices, of the field and the time, are carried out, in general, on relative and non-absolute criteria. Finally, it is necessary to remember that several well delimited and successive studies are preferable with a study of great scale, and that it is important to diffuse the method gradually showing its effectiveness.