## 2 Related Work
[22] also proposed a definition of critical scenario: \"The criticality of a scenario measures the importance in evaluating a performance metric\". Here, the performance is related to the following indicators \"Safety, Functionality, Mobility, Rider's comfort\". Another definition is given by [PERSON] et al. [23], in these words: \"Critical scenarios are defined as scenarios that need to be tested, regardless, whether the requirements are functional or non-functional\". However, the criticality referred to by these authors can only be effectively measured once the simulation has been carried out and the reaction of the AV observed. Furthermore, the number of scenarios that need to be tested is very high and the identification of all critical scenarios can be slow, and yet we have not found in the literature a metric to guide the generation of scenarios before simulation, by identifying the potentially critical scenarios beforehand. In this paper, we thus propose a sensitivity metric to fill this gap. The notion of \"sensitivity\" mentioned here is quite different from the usual meaning, and consists in analyzing how the uncertainty in a system's output (e.g., a failure probability, an injury severity probability) can be attributed to the uncertainty in its input variables [24; 25]. Four key concepts appear in the context of scenario generation for the safety validation of AVs: scenario, scene, situation and event. Definitions have been proposed for each of these by several authors [26; 27; 28; 29]. The desire to establish a consensus has given rise to reflections, notably in the framework of ISO 21448. The definitions proposed by [PERSON] et al. were considered as references in the preliminary version of this standard. However, [PERSON] et al. [30] found that some terms, such as \"scenery\" and \"self-representations\". were still under discussion. They also noted that the definition proposed by [PERSON] et al. for the concept of scene, used to define a scenario, remains unclear: \"A scene describes a snapshot of the environment including the scenery and dynamic elements, as well as all actors' and observers' self-representations, and the relationships among those entities. Only a scene representation in a simulated world can be all-encompassing (objective scene, ground truth). In the real world it is incomplete, incorrect, uncertain, and from one or several observers' points of view (subjective scene)\". This definition highlights that a scene can contain a great deal of information and its implementation can be very subjective and vary from person to person. Furthermore, the term \"_snapshot_\" makes the elements of the environment lose their stable character (no loss of movement or duration of lifespan). As an alternative, [PERSON] et al. [30] took a closer look at the concept of situation. They found that the term \"operational situation\" is often considered in scenarios for describing accidents [31] or in methods for determining \"hazardous events\" [32]. Let us remember that we seek knowledge of the state of the AV and the elements of the situation [33]. We need to know which conditions must be true to solicit a reaction from the AV [28]. As a result, [PERSON] et al. point out that focusing the scenario description on the situation makes it possible to highlight the influencing factors and interactions between the elements of the situation and the AV. This is particularly relevant because the situation also indicates a restrictiveness related to the relevant elements of the scene and is therefore a more interesting level of abstraction than the scene. In this work, the concepts that will be of interest are scenario, operational situation and event. The definitions proposed by [PERSON] et al. serve as a reference to define these concepts [30]:  
Operational situation: set of states of the AV and other entities in an operational environment present for a period of time, defined by the stability of these states. A scenario: description, in a given time interval, of the temporal sequence of operational situations.